Waves,!Sound! and!light!!module! Studio4style!Class

Transcription

1 Next%Generation%Physical%Science% and%everyday%thinking%! Waves,!Sound! and!light!!module! Studio4style!Class!

2 !

3 Next%Generation%Physical%Science% and%everyday%thinking%! Waves,%Sound%and% Light%Module% Unit!WS! Mechanical!Waves! and!sound!!!!!! Studio3style!Class!!!

4 Unit WS: Mechanical Waves and Sound Table of Contents Activity # Activity (A) Title Page A1 Wave Pulses WS-1 A2 Continuous Waves WS-15 Ext A 1 How Do Waves Move? online A3 Two-Dimensional Waves WS-27 Ext C Reflection of 2D Waves online A4 Sound Waves WS-43 Ext D Seismic Waves online A5 ED Engineering Design (forthcoming Summer 2016) WS-63 1 Extensions (Ext s) are online homework activities.

5 UNIT WS Developing Ideas ACTIVITY 1: Wave Pulses Purpose You have probably seen waves on the ocean, on a lake, or even in the bath. These waves are disturbances that move through the water, but similar disturbances can move through other things, such as springs, strings, groups of people, the air and even the earth itself. In this unit we will investigate some of the properties of different types of waves and how we can understand them. The key question for this activity is: What are some different types of wave pulses and what are some of their characteristics and properties? Initial Ideas You will need: Long spring with block attached You have probably seen (or even been a part of) a stadium crowd doing The Wave (sometimes also called the Mexican Wave). Your instructor may even lead the class in a demonstration! What conditions/factors are necessary for such a Wave to be successful? (In thinking about this question it may help to consider what conditions/factors might be missing when a Wave does not happen.) 2016 Next Gen PET WS-1

6 Unit WS Now, place the spring and block on the table and pull on the open end of the spring so that its coils become slightly separated. (If the block moves, you have pulled too hard!) Without letting go of the open end of the spring, what could you do to that end that would result in some sort of disturbance moving along the spring to make the block at the other end move, even if only slightly? Experiment to find as many different ways of doing this as you can. Describe the different ways you found to make a disturbance move along the spring to make the block at the other end move. In each case describe what you did, what the disturbance looked like as it moved along the spring, and how the block moved at the other end. Also draw some pictures to illustrate your methods. Do you think your disturbances carried energy along the spring? If so, what evidence supports your thinking? Participate in a class discussion. Make a note of any ideas that are different from those of your group. WS-2

7 Activity 1: Wave Pulses Conditions for a Wave In the Initial Ideas section you created disturbances that traveled along a spring. The disturbances were examples of wave pulses and the spring was the medium through which these pulses traveled. (The medium for any wave is the material through which it moves. For example, for waves in the ocean the medium is simply the water.) In the discussion the class also probably identified another factor that is necessary for a wave to occur; a source (a way to get the wave started). Another necessity is a mechanism, which is a way for the disturbance to be transmitted/communicated between different parts of the medium. For waves on springs (and strings) the different parts of the medium are physically connected together and so it is effectively a series of contact push/pull interactions between them that is responsible for the transfer of energy. In the rest of this activity you will examine the behavior and properties of wave pulses that move along a spring and a string. Collecting and Interpreting Evidence You will need:! Long spring! Computer with internet connection Exploration #1: What are some different types of wave? STEP 1: The class probably suggested several different ways to create wave pulses in a spring. Scientists generally classify waves into one of two different types, depending on how the parts of the medium move relative to the direction that the disturbance itself is moving. You will now use the large spring to examine these two types of wave. Two members of your group should stretch the spring so it lies along your table with the coils distinctly separated. (Depending on your particular spring, your instructor may give additional instructions.) Now the person at one end should quickly move their hand sideways (at right angles to the length of the spring) and back to its original position. Do it only once for now and this should create a single disturbance (pulse) that moves along the spring. Repeat making these single pulses as necessary to answer the following question. WS-3

8 Unit WS While the disturbance moves along the spring, carefully watch one of the coils near the middle. As the disturbance passes, does the coil move sideways in the same manner that the hand did, or does it move forward, in the same direction that the disturbance is traveling? When the parts of the medium (the coils of the spring in this case) move side to side in a direction that is perpendicular (at right angles) to the direction that the wave that is moving through it, this is called a transverse wave. By moving the end of Pulse moves forward the spring to the side and back you created a single transverse wave Coils move sideways pulse that moved Top view of transverse wave pulse moving along a spring through the spring. (Note that moving the end of the spring upward and then back down would also create a transverse wave pulse, but the coils would be moving up and down instead of side to side.) STEP 2. Now have the person at one end of the spring quickly move their hand forward a few inches (toward the other end of the spring) and back to its original position. This should create a different looking single wave pulse that moves along the spring. Repeat making these pulses as necessary to answer this question. While these pulse moves along the spring carefully watch one of the coils near the middle. As the pulse passes, does the coil move forward and backward like the hand did, or does it move sideways like it did for a transverse wave? When the parts of the medium move back and forth in the same direction as the wave that is moving through it, this is Pulse moves forward Coils move forward and back k Top view of longitudinal (compression) wave pulse moving along a spring WS-4

9 Activity 1: Wave Pulses called a longitudinal wave. (Sometimes also called a compression wave.) By moving the end of the spring forward and back you created a single longitudinal wave pulse that moved through the spring. STEP 3. You have now seen how both transverse and longitudinal wave pulses can move along a spring. Notice that, in both cases, if we focus on the wave pulse itself (the disturbance) we see it move forward through the medium, transferring energy as it goes. But, if we focus on a small piece of the medium (a single coil of the spring), while it does move slightly as the wave passes (side to side or back and forth depending on whether it is a transverse or longitudinal wave pulse), after the wave has passed that piece of the medium is back where it started (or close to it). Thus a wave is a way of transferring energy (and hence information) though a medium without any overall movement of the material itself. In the Initial Ideas section you made a block move without actually touching the block directly yourself. Complete the G/R energy diagram so that it describes how this happened 1. (Note this could apply to both transverse and longitudinal wave pulses.) Complete this statement of conservation of energy for this chain of interactions (assuming no others are occurring at the same time). Decrease in of = Increase in of 1 You may be wondering why no energy changes are shown for the spring in this diagram. After all, as the wave pulse moved along it, different parts of the spring moved and also got stretched and compressed, from which we can infer that there were changes in kinetic energy and elastic potential energy. However, after the pulse had passed, each part of the spring was both at rest and back where it started, so all the changes were temporary and at the end there was no net change in either of these types of energy. WS-5

10 Unit WS Exploration #2: What are some characteristics of wave pulses on a string? STEP 1: Open the simulator link for this activity (UWS-A1 - Sim). It shows a straight string on which waves can be generated by a source that moves the left end of the string up and down. (This would be like you moving the end of your spring up and down.) Note that the string is represented as a series of dots this is done to make it easier to follow the motion of one particular part of the string if necessary. (Imagine a transparent string with a regular series of dots painted on it.) Before proceeding, set the simulator to Pulse mode and make sure that the string is set to have No End. Use the sliders to set both the Amplitude and Pulse Width to 100, and set the Damping to 0. Also, set the tension (how strongly the string is being pulled lengthways) to about 70% of its maximum. STEP 2: Now click the Pulse button. The source should move up and then immediately back down to its starting point, thus generating a single pulse that moves along the string from left to right. Create more pulses as necessary as you answer the following questions. As the wave pulse moves from left to right, how does each part of the string (each dot) move; up and down or forward and backward? Does this mean the wave pulse is transverse or longitudinal in nature? The dashed line that is revealed as the pulse moves along represents the position of the string before it was disturbed and is called its equilibrium position. Scientists call the maximum distance the parts of the medium move away from its equilibrium position, the amplitude of the pulse. Thus, for the pulse on the string in the simulator the amplitude is simply the height of the pulse. Amplitude of pulse Equilibrium position WS-6

11 Activity 1: Wave Pulses Does the amplitude of a pulse change or stay constant as it moves along the string? We also use the term amplitude to refer to the maximum distance the source mechanism moves away from its equilibrium position as it generates a wave pulse. Amplitude of source Generate some pulses with different amplitudes (adjust the Amplitude slider) and look at how the amplitude of the source compares with the amplitude of the wave pulse it generates. (You may find it useful to display the Rulers tool and move the horizontal ruler to help judge the relative amplitudes of the source and pulse.) Is the amplitude of a wave pulse approximately the same as the amplitude of the source that generates it, or is it very different? STEP 3. In reality the transfer of energy from one part of a string to the next is not done with 100% efficiency. (Some energy goes to warming the string as different parts of it rub together internally.) The Damping control in the simulator sets to what degree this happens. Currently it should be set to zero, meaning no energy is transformed to thermal energy as a wave pulse moves along the string. Suppose you were to increase the Damping control in the simulator to something above zero, what would you expect to happen to the amplitude of a wave pulse as it moves along the string? Briefly explain your thinking. Check your thinking by increasing the amount of Damping present in the simulator. Create three new pulses, making sure they are separated by a short distance, and then pause the simulator while all of them are still visible on the string. WS-7

12 Unit WS Sketch the string below. (Draw a continuous line rather than individual dots.) Indicate the amplitude of each pulse as shown in your picture. What happens to the amplitude of a pulse as it moves along the string now? Why does it make sense that this happens? STEP 4. Another property of a wave pulse is its width, which we can regard as how wide it is across its base. Currently the Pulse Width control in the simulator should be set to a value of 100. Let us look at how pulses of different widths are generated by the source. Pulse width First reset the Damping control to zero (While this is somewhat unrealistic, it will make the simulator results easier to interpret.) Now generate some pulses with different widths (adjust the Pulse Width slider) and look at how quickly the source moves up and down when producing these different widths. How is the up and down speed of the source different when it is generating a narrow pulse as opposed to a wider pulse? Why does it make sense that the longer it takes for the source to move up and down, the wider the pulse it generates becomes? (Hint: Consider how far the front of the pulse created by the source moving upward will have moved along the string by the time the back of the pulse gets created by the source coming back down.) WS-8

13 Activity 1: Wave Pulses Exploration #3: What determines the speed of a wave pulse? STEP 1. You will now use the simulator to investigate what factors affect the speed of a wave pulse. By this we mean the speed at which the pulse disturbance moves along the string, NOT the speed at which a particular part of the string (a single dot) moves up and down as the pulse passes. First, consider what you think would happen if the amplitude and/or the width of the pulse were changed. Do you think pulses with different amplitudes and/or widths would move along the string at different speeds or not? Why do you think so? Rather than trying to measure the speed of pulses in the simulator, consider another way we could check. Suppose two pulses of different amplitudes were moving to the right along the string, as shown here. Consider what would happen to the distance between these two pulses as they move further along the string if they moved at different speeds. If pulses with larger amplitudes had a higher speed would these two pulses get further apart, closer together, or stay the same distance apart as they moved along the string? If pulses with larger amplitudes had a slower speed would these two pulses get further apart, closer together, or stay the same distance apart as they moved along the string? WS-9

14 Unit WS STEP 2. Click the Restart button and then create two pulses with significantly different amplitudes (but the same width), so that they follow each other along the string. Note: A convenient way to do this is: Create the first pulse and immediately pause the simulator. Now adjust the amplitude control and click the Pulse button again. Click the pause/play button, and the second pulse should then be created. Repeat as necessary as you answer the question below. Remember, you can pause the simulation at any point to more easily check the distance between the pulses. Does the distance between the pulses change significantly as they move along, or does it stay about the same. What does this tell you about whether or not the speed of a wave pulse depends on its amplitude? Now use the same technique to create two pulses with significantly different widths (but the same amplitude), one straight after the other. Does the distance between the pulses with different widths change as they move along, or does it stay about the same. What does this tell you about whether or not the speed of a wave pulse depends on its width? WS-10

15 Activity 1: Wave Pulses STEP 3. In order to have a string be straight it must be pulled tight from end to end. The strength of the force that is being used to pull it tight is called the tension of the string. (Many stringed musical instruments have pegs that are used to adjust the tension of their strings.) Do you think changing the tension of the string would affect the speed at which waves move along it? Briefly explain your thinking. Again, check your thinking using the simulator. Create a single pulse and adjust the tension in the string as the pulse is moving along it. Repeat as necessary as you answer the question below. Does increasing the tension of the string make the wave pulses move faster, slower, or does it not affect their speed? Check your ideas about which factors (amplitude, width, tension) affect the speed of a wave pulse with at least two other groups. Try to resolve any differences. As you have now seen, changing the motion of the source of a wave pulse, so making pulses that are wider/thinner or higher/lower, does not change the speed at which the pulses it generates move through the medium. Instead, in general, the speed of waves that move through a physical medium (which are called mechanical waves) is determined only by the properties of the medium itself. As you have seen, for transverse waves on a string the speed is determined by the tension of the string. (Another factor that affects the speed of waves on a string is how heavy each section of the string is, which is related to how thick the string is. This is why stringed instruments have strings of various thicknesses.) WS-11

16 Unit WS STEP 4. If they are not already, reset the simulator controls to the values you started with in Exploration #1. ( Amplitude = 100, Pulse Width =100, Damping = 0, tension = 70% of maximum.) Generate a pulse and then pause the simulator. You should find that the width of the pulse is about 9 to 10 dots. Now, suppose you were to reset the simulation, increase the tension of the string to 100% (without changing any other settings), and then generate a new pulse. Do you think the width of this new pulse would be the same as the previous one, or would it be wider or narrower? Why do you think so? Check your thinking by clicking on the Restart button, increasing the tension in the string to its maximum value and then generate a new pulse. How does the width of this new pulse compare with the previous one? Does this agree with your prediction above? Why does it make sense that, even though nothing about the motion of the source has changed, when you increase the tension in the string the pulses generated by the source become wider? (Hint: Consider what effect increasing the tension has on the speed with which a disturbance moves along the string, and how far the front of the pulse will have moved along the string by the time the back of the pulse gets created.) WS-12

17 Activity 1: Wave Pulses Exploration #4: What happens when a wave pulse reaches the end of the medium? STEP 1: Up to now we have set our simulator string to have No end, but of course, in reality, every string must end somewhere. What do you think will happen to a wave pulse when it reaches the end of the medium through which it is moving? Will it simply disappear, or will something else happen and, if so, what? Stretch out your long spring as you did in Exploration #1. Have one group member create some transverse and longitudinal wave pulses at one end of the spring and watch what happens when they reach the other end. When both the transverse and longitudinal wave pulses reach the other end of the spring, do they just disappear, are they reflected back again, or does something else happen. (If the latter, describe it.) To support your observations, return to the simulator and select the Fixed End option. The right end of the string is now clamped in such a way that it cannot move. Create a wave pulse and watch as it moves to the other end of the string. Describe what happens to the wave pulse when it reaches the fixed end of the string. Instead of being clamped firmly in place the end of the string could be allowed to move up and down. In the simulator select the Loose End option. The right end of the string is now attached to a ring that is free to move up and down on a rod. Again, create a wave pulse and watch as it moves to the other end of the string. Describe what happens to the wave pulse when it reaches the loose end of the string. WS-13

18 Unit WS You have now seen that when the wave pulse reaches the end of the string it is reflected back along the string in the opposite direction. If the end is fixed (non-movable), the reflected pulse is upside-down. If the end is loose (free to move), the reflected pulse is right side up. Summarizing Questions S1: You have seen that as a wave pulse moves it transports energy through the medium. If you were to increase the amplitude of a wave pulse what effect do you think this would have on the amount of energy associated with it? Give a real-world example to illustrate your answer. S2: Imagine all the groups in your class have identical ropes. You then have a race between pulses that each group creates at one end of the rope at the same time. What should your group do to maximize your chances of your pulse winning this race? Why would this work? S3: In the race described in the previous question two groups (A and B) create their pulses in an identical manner. That is they move one end of the rope up and down by the same amount in the same amount of time. However, Group A is pulling their rope tighter than the Group B. On which rope (A or B) will the generated pulse have a bigger width? Explain why this is. WS-14

19 UNIT WS Developing Ideas ACTIVITY 2: Continuous Waves Purpose In the previous activity you examined the behavior of single wave pulses on a spring and a string. If the source that created these pulses were to keep repeating exactly the same motion over and over again, it would create a series of identical pulses that move through the medium. We call such a series of identical pulses a continuous wave and in this activity you will examine these in more detail. The key question for this activity is: What are some characteristics and properties of continuous waves? Collecting and Interpreting Evidence You will need: Long spring Access to computer with internet connection Exploration #1: What are some characteristics of continuous waves? STEP 1. In the previous activity you created single wave pulses on a spring in two ways. You created a transverse wave pulse by moving the end of the spring to the side and then back again. If you were to keep repeating this motion, moving your hand side-to-side in the same manner over and over again, you would create a series of pulses that would move along the spring. Stretch the spring out along the table (or floor) as you did in the previous activity and try this now Next Gen PET WS-15

20 Unit 6 The pattern you see on the spring while you are doing this should look something like that shown below. It consists of a regular series of identical pulses that move along the spring as a continuous transverse wave. Continuous transverse wave on a spring Similarly, in the previous activity you created a longitudinal wave pulse by moving the end of the spring forward and then back again. Stretch the spring along the table and keep repeating this motion to create a continuous longitudinal wave. Continuous longitudinal wave on a spring STEP 2. You will now use the same Waves on a String simulator to examine some properties of continuous waves. Open UWS-A2 - Sim and select Oscillate as the source of the wave in the simulator. This will introduce a mechanism that moves the end of the string equal distances up and down repeatedly in a consistent manner. Also set the string to have No End. Next, set the controls at the top of the window as shown below. (Amplitude = 25, Frequency = 25, Damping = 0, tension = 80% of maximum.). After a few seconds there should be a continuous transverse wave moving along the string. (Note that the source now moves both above and below its equilibrium position, as does each part of the string.) WS-16

21 Activity 2: Continuous Waves Pause the simulation and sketch the string to show a snapshot of the continuous wave you have created. (Just draw a continuous line rather than the serious of dots shown by the simulator.) We will call the highest points of such a wave (above the equilibrium position) peaks and the lowest points (below the equilibrium position) valleys. Mark all the peaks on your sketch with a P and all the valleys with a V. STEP 3: Now try varying the amplitude and frequency controls and see what effect they have on the motion of the oscillating source (do not look at the wave for now). How does the motion of the oscillating source (not the wave it creates) change as you increase and decrease the amplitude? How does the motion of the oscillating source (not the wave it creates) change as you increase and decrease the frequency? The frequency of the source is a measure of how often it completes one full cycle of its motion. The value of the frequency is actually a count of how many cycles it completes in a particular period of time. Usually this fixed period is taken to be one second and then the frequency is given units of Hertz (Hz). Thus, a source that has a frequency of 5 Hz is completing five up and down cycles in every one-second period. However, in the simulator the motion of the oscillating source has been slowed considerably to make its motion easier to follow, so the frequency shown is the number of cycles it completes in a much longer period. (Often, the letter f is used to represent frequency.) WS-17

22 Unit 6 Let us now consider what effect varying the amplitude and frequency of the source have on the continuous wave that it is generating. STEP 4. Recall that in the previous activity we defined the amplitude of a wave pulse to be the maximum distance any part of the string moves away from its equilibrium position as the wave pulse passes. We can similarly regard the amplitude of a continuous wave as the height of the peaks above the equilibrium position (which is the same as the depth of the valleys below the equilibrium position). Does the amplitude of the wave seem to be the same as, or different from, the amplitude of the source that is creating it? (If you are not sure, click the Rulers check box and move the horizontal ruler so its edge lines up with the top of the peaks.) Sketch a snapshot of the wave on the simulator and mark the amplitude of the wave at two peaks and two valleys. As well as the source oscillating up and down (with the frequency you have set), each part of the string is also oscillating up and down. In order to measure the frequency of both the source and some parts of the string, click the Timer checkbox to display a timer tool. Set the Frequency slider of the oscillating source to a value of about 10. (The exact value is not important as long as it is low enough that you can follow its motion.) Start the timer just as the source reaches the top of its motion and stop it after it has completed ten whole down and up cycles. Approximately how long does it take the source to complete ten cycles? Now focus your attention on one particular part (one particular dot) of the string. (You may find that one of the green dots is easier to follow.) Reset the timer; start it just as your dot reaches the top of its motion and stop it after it WS-18

23 Activity 2: Continuous Waves has completed ten whole down and up cycles. Make a note of the timer reading, reset it, and repeat this for some other parts of the string. Answer the following questions based on your results. Does the amount of time its takes each part of the string to complete ten down and up cycles seem to be close to the same as that for the source, or is it very different? What does your answer to the previous question imply about the frequency with which each part of the string is oscillating up and down? Is it the same as, or different from, the frequency at which the source is oscillating up and down? Since the string is attached to the oscillating source, each part of the string moves up and down at the same frequency and amplitude (assuming there is no damping) as the source. Thus, we can also talk about the frequency and amplitude as a characteristic of a continuous wave itself. For example, when a continuous wave is being produced by an oscillating source that has an amplitude of 3 cm and frequency of 5 Hz, we can also say that the wave it generates also has an amplitude of 3 cm and a frequency of 5 Hz. STEP 5: Return the amplitude and frequency controls to their original values (25 each) and wait a few seconds for a regular pattern to form again. As the wave moves along the string it traces out a repeating pattern of peaks and valleys. Pause the simulation, check the Ruler s box (if it is not already), and use the horizontal ruler to answer the following questions about this pattern. Does the distance between successive (neighboring) peaks seem to be about the same or is it very different depending on which two successive peaks you look at? WS-19

24 Unit 6 Does the distance between successive (neighboring) valleys seem to be about the same as that between successive peaks, or is it very different? Another characteristic of a continuous wave is its wavelength. This is a measure of the distance between two successive, but otherwise identical, points on the wave; for example, the distance between two successive peaks or two successive valleys. (Often the greek letter lambda (λ) is used to represent wavelength.) Providing the continuous wave is a series of identical pulses any of these ways of measuring the wavelength should give the same result. STEP 5: Though we have defined all of these characteristics from examining continuous transverse waves, they can also be used to characterize continuous longitudinal waves in a very similar way. In this case the amplitude of the wave is the maximum distance a part of the medium (such as a single coil on the spring) moves forward or backward from its equilibrium position and the frequency is how many times it moves backward and forward in one second. For a longitudinal wave the wavelength is the distance between two successive regions of compression or extension (stretching). WS-20

25 Activity 2: Continuous Waves Exploration #2: What determines the wavelength of a continuous wave? STEP 1. You have seen it is the amplitude and frequency of the oscillating source that determines the amplitude and frequency of the continuous wave it creates. However, what determines the wavelength of such a wave? Do you think changing the amplitude of the wave will change its wavelength? Briefly explain your thinking. Do you think changing the frequency of the wave will change its wavelength? Briefly explain your thinking. You will now check your thinking using the simulator. Without changing anything else, create some waves with different amplitudes. For each one, pause the simulation and use the Rulers tool to measure their wavelength. When the amplitude of the wave is changed, what happens to its wavelength (if anything)? Now create some waves with different frequencies (keeping everything else the same). If you are not sure about what effect this has on the wavelength you can again pause the simulation and use the ruler to measure the wavelength. When the frequency of the wave is increased, what happens to its wavelength, (if anything)? WS-21

26 Unit 6 STEP 2. Let us now consider why this relationship between wavelength and frequency makes sense. To do so it may help to consider an analogy. Consider a situation in which a series of identical cars leave a depot at regular time intervals, say every 30 seconds, and drive along the same long straight road at the same speed, say 30 mph. This would result in a line of cars moving along the road which are all the same distance apart. An observer standing by the side of the road would also see one car go by every 30 seconds. In this analogy the depot represents the source of a wave, and the frequency of this source is represented by how often cars leave the depot, which is one every 30 seconds. The frequency of the wave is represented by how often an observer sees a car go by (also one every 30 seconds). The cars themselves represent the peaks of a wave and the distance between them represents the wavelength. Now suppose the cars were to leave the depot more often, say one every 10 seconds, but still drive at the same speed. Since the cars are leaving the depot more often this is analogous to a source with a higher frequency. (It happens more often.) The observer would now also see one car go by every 10 seconds, representing a wave with a higher frequency. Under these circumstances would the distance between successive cars be greater than, smaller than, or the same as it was before? Explain why is this in terms of how far each car would travel before the next one left the depot. What does your answer to the previous question suggest about what happens to the wavelength of a wave when the frequency is increased. Would the wavelength be longer, shorter, or remain the same? WS-22

27 Activity 2: Continuous Waves Explain why this is in terms of how far each peak travels before the next one is created. STEP 3. Finally, consider what would happen if the speed of a wave were changed (by changing something about the medium). Would this have any effect on its wavelength or not? To think about this, consider the line of cars analogy again. In this analogy the speed of the cars represents the speed of the wave. Suppose the cars were again to leave the depot once every 30 seconds, but drive along the road a constant speed of 60 mph (instead of 30 mph as before)? Would the distance between the cars be less than, greater than, or the same as when they drove at 30 mph? Explain why in terms of how far each car travels before the next car leaves. You determined in the previous activity that the speed of waves on a string depends on the tension in the string. (How tightly it is pulled end-to-end.) Suppose you were to increase the tension in the string in the simulator, would you expect the wavelength of the wave to increase, decrease, or stay the same? Why do you think so? To check your thinking reset the simulator amplitude and frequency to the original values (25 each) and generate a continuous wave. Then increase the speed of the waves by increasing the tension in the string to its maximum value. Does the wavelength behave as you predicted? If not, try to explain what does happen. WS-23

28 Unit 6 Relating wavelength to speed and frequency You saw in the previous activity that the speed at which waves move through a medium is determined only by the properties of the medium. (For waves on a string the relevant properties are the tension and the thickness of the string.) Also, the frequency of a wave is completely determined only by the motion of the source of the wave. Whatever frequency the source oscillates at will be the frequency at which the different parts of the medium oscillate as the waves pass. However, you have now seen that the wavelength of a continuous wave depends on both the frequency and the speed. (So it depends on both source and the medium.) When the frequency of a wave is increased (by increasing the frequency of the oscillating source) does the wavelength increase or decrease? When the speed of a wave is increased (by changing some property of the medium, e.g. the tension in a string) does the wavelength increase or decrease? Careful measurements would show us that the wavelength of a continuous wave is related to its speed and frequency by the expression 1 :!"#$%$&'(h!!"!!"#$ =!"##$!!"!!"#$!"#$%#&'(!!"!!"#$ Explain how this relationship is consistent with your answers to the two questions above. We will call all the ideas we have developed about waves in the first two activities of this unit, together with the expression above, our model for continuous mechanical waves. 1 In this expression, if speed is measured in units of meters per second (m/s) and frequency in Hertz (Hz), then the wavelength will be in units of meters (m). WS-24

29 Activity 2: Continuous Waves Summarizing Questions S1: A boy and girl stretch a long rope between them in mid air. Their pull results in a tension that makes the wave speed 4.5 meters per second (m/s). The girl then moves her end of the rope up and down with an amplitude of 20 cm and at a frequency of 3 Hz (three times per second). a) Calculate the wavelength of the continuous wave created by the girl s motion. b) If the girl were to change the amplitude of her up and down motion to 40 cm, but keep the frequency at 3 Hz, would the wavelength of the wave now be less than, equal to, or greater than the value you calculated above? How do you know? c) If the girl were to move her end up and down at a frequency of 6 Hz, would the wavelength of the wave be less than, equal to, or greater the value you calculated in part a)? Explain how you know. WS-25

30 Unit 6 d) Suppose the girl again moves her end of the rope up and down at a frequency of 3 Hz, but the boy at the other end pulls harder on his end (increasing the tension in the rope). Would the wavelength of the wave now be less than, equal to, or greater than the value you calculated in part a)? Explain how you know. WS-26

31 UNIT WS Developing Ideas ACTIVITY 3: Two-Dimensional Waves Purpose In the previous activities you developed a model for continuous mechanical waves by examining waves that move along a string or a spring. The nature of the medium (the string or spring) in these cases meant that these waves were only one-dimensional (that is they only moved either forward or backward) and did not spread out in different directions. However many types of waves we encounter are two (or even three) dimensional. For example, ripples on the surface of a pond spread out across the surface in all directions and earthquake waves spread out through the body of the Earth. In this activity we will examine some of the characteristics of twodimensional waves by looking at the behavior of surface waves on water under different circumstances. What are some characteristics of twodimensional waves? Initial Ideas While trying to feed the ducks at the park you throw a large number of breadcrumbs that spread out over the surface of the lake. Before the ducks arrive a child throws a stone into the water in the middle of the breadcrumbs. The stone creates ripples on the surface of the water that spread out away from where the stone entered and make the breadcrumbs bob up and down. While this is happening you notice that the amount that the breadcrumbs bob up and down gets less and less the further they are from the center of the ripple pattern Next Gen PET WS-27

32 Unit WS Why do you think the amplitude with which the water goes up and gets less and less as the ripples spread out? Draw diagrams and write some sentences to explain why you think this happens. Participate in a class discussion abut your ideas and make a note of any other ideas expressed by the class that make sense. Collecting and Interpreting Evidence WS-28 You will need: Computer with internet connection Exploration #1: How can we represent water waves in two dimensions? STEP 1. You know that when the surface of a body of water is disturbed in some way this creates waves (ripples) that move across the surface. To create continuous waves on the water surface you simply need to disturb it in a regular repeating pattern. The picture here shows one way to visualize such a situation. In effect it shows a side view of a thin slice of water with a continuous series of ripples moving to the right. These were created by a regular series of drips falling into the water on the left side. Since these drips only disturb the water in one particular place we call them a point source. To see a dynamic version of this representation, open UWS-A3 - Sim. (This is another of the PhET simulators.) When the simulator first opens you need to adjust the settings to show the side view.

33 Activity 3: Two-Dimensional Waves Pause the simulator using the button at the bottom of the window and make sure the Water tab is selected at the top of the window. Set the Rotate View slider control on the right to the Side setting The Frequency and Amplitude sliders control how often the drips of water occur and how large the drops are. For now, set them to somewhere near the mid-point of their ranges. Finally, restart the simulator using the button at the bottom of the window. You should now have a side view similar to that in the picture on the previous page, except that everything is now in motion. Do you think the waves that move across the surface of the water are transverse or longitudinal in nature? Why do you think so? To check your thinking open another browser window, return to the Student Resources page, and watch UWS-A3 - Movie 1, which shows a ball floating in deep water. As wave disturbances move across the surface water does the ball continue moving forward with them or does it mostly bob up and down in approximately the same location? WS-29

34 Unit WS Assuming the ball is showing the movement of the water itself, does this seem to indicate that water waves are transverse or longitudinal in nature 1? Now return to the simulator. Notice that from this view the surface of the water looks very much like the shape of a string as transverse waves move along it. As the ripples move further from the source (the drips) what happens to their amplitude? Why do you think this is? Try varying the frequency of the water drops. What happens to the wavelength (distance between successive peaks) of the ripple pattern as the frequency is increased? Explain why this is, in terms of how far the previous wave peak travels by the time the next one is created. Also explain how this behavior is consistent with this relationship, introduced at the end of the previous activity.!"#$%$&'(h!!"!!"#$ =!"##$!!"!!"#$!"#$%#&'(!!!!!"#$ STEP 2. Because this side view representation looks at only a single slice of water, it does not allow us to examine the two- 1 As the video shows, in deep water surface waves are almost purely transverse in nature. However, when the water is very shallow, frictional effects between the water and the solid bottom cause these waves to have a longitudinal component also. WS-30

35 Activity 3: Two-Dimensional Waves dimensional nature of these ripples. To see an alternative top view representation, move the Rotate View slider to the Top setting. Describe how this view represents the peaks and valleys in the water surface. What do the brighter regions represent, peaks or valleys? What about the darker regions? (If you are not sure, pause the simulation and switch back and forth between the Side and Top views.) How does this representation show peaks and valleys that have different amplitudes? (If you are not sure, try varying the amplitude of the source.) How can you tell from this representation that the amplitude of the ripples decreases as they get further from the source? Recall that the wavelength of a wave is the distance between two successive peaks (or two successive valleys). How could you measure the wavelength of the waves in this top view representation? Show your thinking by marking a single wavelength (λ) at two different places on this picture. (One using peaks and the other using valleys.) WS-31

36 Unit WS Pause the simulator and use the Measuring Tape tool to measure the wavelength. Record the value here. STEP 3. As you probably deduced, the brighter regions in this representation show where the peaks in the ripple pattern are and the darker regions show the valleys. (In other words they show where the water surface is above and below its equilibrium position respectively.) The curved region where a peak is located in a two-dimensional wave is called a wavefront and the simulator representation shows these wavefronts as bright regions moving away from the source and spreading out as they do so. Shown here is a representation of a set of circular ripples as they move away from the source in the middle. Sometimes it is useful to add some arrows to this representation to show what direction the wavefronts are moving. One such arrow has been drawn to show that along that line the wavefronts are moving to the right. Note that at each point the arrow is perpendicular (at right angles) to the curved wavefront. Draw several other arrows on this diagram to show what direction the wavefronts are moving on different parts of the pattern. For the one-dimensional waves on a string, if there was no damping, the amplitude of the wave stayed constant as it moved along the string. While there maybe some damping in water waves also, even without this the amplitude of the ripples would still decrease as they move further away from the source. Let us consider why this is. Because the source is disturbing the water by the same amount with each drip, each ripple carries the same amount of energy with it. However, as each ripple moves away from the source, its circumference (distance around the complete circle) gets larger and larger. WS-32

37 Activity 3: Two-Dimensional Waves Use these ideas to explain why, even though it carries the same amount of energy in total, the amplitude of each ripple gets smaller and smaller the further it gets from the source. WS-33

38 Unit WS Exploration #2: What happens when the speed of 2D waves changes? STEP 1. Up to now we have used point sources (single drips) to generate circular ripple patterns. However, it is also possible to have other shape sources. For example if we were to dip a pencil in and out of the water lengthways we would have an extended line source that would produce a ripple pattern like that shown to the right. The direction arrows drawn on the pattern show that across a region in front of the source the wavefronts are straight lines all moving in the same direction. STEP 2. Earlier in this unit you saw that the speed with which waves move on a string depends on how tightly the string is pulled (its tension). For waves on the surface of water it is the depth of the water under them that determines the speed, but which way is it; does deeper water mean faster or slower surface waves? Below are top views of two water tanks (A and B). Both tanks have the same depth in the lower half, in which identical ideal line sources are located. In these pictures the simulator has been running for only a short time and the wave fronts generated by these sources have only moved a short distance away from the source. Since everything is the same in the lower half, the waves generated in both tanks travel at the same speed and have the same wavelength in this section. WS-34

39 Activity 3: Two-Dimensional Waves However, in both tanks the waves are headed toward the other (upper) half of the tank. In Tank A the water in the upper section is deeper than in the lower section. In Tank B the water in the upper section is shallower than in the lower section. The leading wavefronts will reach the half-way point in each tank at the same time. Play UWS-A3 Movie - 2 to see what happens when the waves pass into the upper section of the two tanks. (A third tank, in which the depth is the same throughout, is also shown.) Some strange effects occur due to some of the wave energy being reflected back toward the bottom of the tank at the halfway mark, but you can ignore this. Instead focus your attention on the waves that do cross into the upper section and continue moving toward the top of the tank. (We say they are transmitted across the boundary between the two sections.) Pay particular attention to the speed with which the wavefronts move in the top section of each tank. (One easy way to compare speeds is to look at how far each set of waves has moved past the boundary by the time the movie stops.) When the water gets deeper, does the speed of the waves increase, decrease, or stay the same? What about when it gets shallower? What happens to the wavelength of the wave when it passes into the deeper water? Does it get longer, shorter, or remain the same? How can you tell from the wavefronts displayed on the simulator? What happens to the wavelength of the wave when it passes into the shallower water? Again, how can you tell from the wavefronts? WS-35

40 Unit WS Why do these changes in wavelength make sense given what you know about how the wavelength of a wave depends on its speed and frequency 2?!"#$%$&'(h!!"!!"#$ =!"##$!!"!!"#$!"#$%#&'(!!"!!"#$ STEP 3: In the previous step the wavefronts arrived head-on at the boundary between regions of different depth and kept going in the same direction after crossing the boundary. What would happen if they arrived at this boundary at a different angle? The pictures below show the same two tanks (A and B), except that the line sources have now been tilted so that the wavefronts they generate will encounter the boundary at an angle, rather than head-on. (The arrows show the direction the wavefronts will be moving as they arrive at the boundary.) 2 Our ideas about conservation of energy tell us that the rate at which energy arrives at any point in a medium must be equal to the rate at which it leaves that same point. (Providing there is not an energy giver or receiver at that point.) Since each pulse of a wave carries a fixed amount of energy, the law of conservation of energy tells us that the frequency of a wave does not change when it crosses a boundary. WS-36

41 Activity 3: Two-Dimensional Waves After they cross the boundary, you now know that the speed (and hence wavelength) of the transmitted waves will be different, but do you think will they still be moving in exactly the same direction or not? Why do you think so? To see what happens, watch UWS-A3 - Movie 3. Again, ignore the effects that occur in the lower half of each tank because some of the wave energy is reflected back from the boundary. Instead look at the transmitted waves in the upper half of both tanks. The pictures below are taken from the simulator after it has been running for a few seconds. Arrows have been added to show the direction that the transmitted wavefronts are moving. Is the direction of motion of the transmitted wavefronts exactly the same as their direction before they crossed the boundary or did they change direction, even if only slightly? WS-37

42 Unit WS This phenomenon, in which two-dimensional waves change direction when they cross a boundary in a medium in which their speed is different, is called refraction. (You may learn more about refraction if you study light in the next unit.) Notice that when the wavefronts crossed to a region in which their speed is greater (Tank A) they turned slightly in one direction. However, when they crossed to a region in which their speed is less (Tank B) they turned slightly to the opposite direction. Let us try to understand why this is. STEP 4. One way to think about why refraction occurs is to use an analogy of a toy car that is rolling down a ramp and crosses from one surface to another at an angle. This means that the wheels on the axle cross the boundary at different times. Consider what would happen to the car if, as each wheel crosses the boundary, its speed changes. To the right is a top view of the front axle and wheels of such a toy car as its left wheel is about to cross the boundary between a hard surface and a soft surface. Assume that the soft surface causes a greater frictional effect, so that the speed of each wheel decreases to half its L previous value as it crosses the boundary. R Soft (slower) Hard (faster) In the time it takes the right wheel to reach the boundary how far do you think the left wheel will have moved; farther than, less than, or the same distance as the right wheel? Why is this? WS-38

43 Activity 3: Two-Dimensional Waves On the diagram on the previous page, draw what you think the approximate position of the two wheels will be a short time later, just as the right wheel is about to cross the boundary. Join these two positions with a straight line representing the axle and also draw an arrow showing in what direction you think the car is now headed. In this case, when the left side of the car is the first to cross a boundary to a surface for which the speed is less, would you expect the car to turn slightly to the left, or slightly to the right? Repeat this analysis for a toy car as its left front wheel is about to cross a boundary between a soft surface and a hard surface. Assume that the hard surface causes a smaller frictional effect, so that the speed of each wheel increases to Hard (faster) Soft (slower) double its previous L value as it crosses the boundary. R In this case, when the left side of the car is the first to cross a boundary to a surface for which the speed is greater, would you expect the car to turn slightly to the left, or slightly to the right? Explain your reasoning. To check your thinking, watch UWS-A3 - Movie 4, which shows a toy car rolling down a ramp that has two different surfaces a hard board (faster speed) and softer carpet (slower speed). WS-39

44 Unit WS Describe how the toy car behaves as it crosses the boundary in both cases (Fast to slow, and slow to fast). Is this behavior what you predicted above? STEP 5. This toy car analogy should help you understand why water waves change direction when they cross a boundary at an angle into a region in which their speed is different (refraction). Deep Briefly explain why water waves turn as they do when they cross the boundary shown in the diagram above. Shallow Summarizing Questions S1: A swimming pool has a gradually changing depth from one end to the other. To divide the pool into two areas a lifeguard throws a rope across it at exactly halfway along its length. This creates some straight wavefronts that travel toward each end of the pool. A diagram of this situation is shown on the next page. (Recall that the frequency of a wave is determined by the source that creates it, so the frequency of the waves created in both halves of the pool will be the same.) WS-40

45 Deep Rope Activity 3: Two-Dimensional Waves Shallow a) Which end of the pool will the wavefronts reach first, and why? b) At which end of the pool will the wavelength be shorter, or will they both be the same? Explain why. S2: Use the toy car analogy to explain why the wavefronts of a water wave do not change direction when crossing a boundary head on into a region in which their speed is different. WS-41

46 Unit WS S3: In the diagram in STEP 5 of Exploration #2 the? wave passes from shallow into deeper Shallow water. Now, suppose instead the same wave was to pass from deep Deep into shallower water. Use this picture to show how the wavefronts would behave now as they cross such a boundary and write a few sentences to explain your reasoning. (Pay attention to both the wavelength and direction of the transmitted wave.) WS-42

47 UNIT WS Developing Ideas ACTIVITY 4: Sound Waves Purpose Sound is all around us every day, from the background noises of everyday life, to the conversations we have with other people, and the music we listen to. But what is sound? How is it produced and detected, and how does it travel from the source to the receiver? You are probably aware that sound can be regarded as a wave of some sort, but in this activity we will take a closer look at these waves. What are sound waves and what are some of their characteristics? Initial Ideas Your instructor will connect a bell to an electrical source and let it ring (or show you a videoof this). Write several sentences to describe what you think is happening that allows people to hear the sound of the bell. Also draw and label a diagram to illustrate your thinking 2016 Next Gen PET WS-43

48 Unit WS If the ringing bell were placed in a sealed container do you think you it could still be heard? Why or why not? Now suppose most of the air was removed from inside the sealed container, with the ringing bell still inside, do you think it could still be heard as easily? Why or why not? Participate in a class discussion about your ideas and predictions. You instructor will now place the ringing bell in a sealed container and use a pump to remove the air (or show you a video of the experiment being done.) Before the air was removed from the container, can the bell still be heard? What about after most of the air was removed? If the results do not agree with your prediction, make a note of any different ideas the class suggests that might be useful? WS-44

49 Activity 4: Sound Waves Collecting and Interpreting Evidence Exploration #1: What are sound waves? You will need:! Ruler! Tuning fork and mallet (optional, may be shared with other groups)! Computer with internet connection STEP 1: Hold the ruler flat on the table so that part of it is hanging over the edge. Now give the free end of the ruler a quick flick so that it begins oscillating up and down. (If you do not hear a sound, adjust how much of the ruler is hanging over the edge of the table.) What do you think is happening to make the sound that you hear? Why do you think the ruler creates a sound when it is moving up and down but not when it is stationary (or moving in only a single direction)? If a tuning fork is available to you, hold the handle end in your hand and strike one of the prongs firmly with the mallet. Otherwise watch UWS-A4 - Movie 1. What is it about the tuning fork that produces the sound you hear? To check your thinking gently touch one of the prongs while you can still hear the sound. What do you feel with your finger when you do this? Is this what you expected? WS-45

50 Unit WS As you are probably aware, sound is produced when objects vibrate. The movement of the vibrating source affects the air around the object to produce sound waves. One common way to do this is using a device we call a loudspeaker in which an oscillating electrical current (which is another example of a wave) is passed through a coil. Because of the electromagnetic interaction between them, a magnet located in the center of this coil then vibrates back and forth and so the cone attached to this magnet also vibrates, producing sound waves. You will now use a simulation to examine the nature of these sound waves. STEP 2. Open UWS-A4 - Sim. (This is the same PhET simulator you used in the previous activity.) Before starting to use it, you need to make some adjustments to the default setup. First, select the Sound tab at the top of the window. The window should now show a single loudspeaker. Make sure the Grayscale option is selected. Adjust the frequency slider it is only a short way along from the left. Adjust the amplitude slider so it is at its maximum (all the way to the right). The loudspeaker should now be moving forward and backward, creating sound waves that you can see moving through the air away from the loudspeaker. (This representation is similar to that you saw for water waves in the previous activity.) Does the motion of the loudspeaker suggest that the sound waves it is creating are transverse or longitudinal? Briefly explain your thinking. WS-46

51 Activity 4: Sound Waves STEP 3: Let us now examine the mechanism by which the sound waves created by the loudspeaker move through the air. As you are probably aware air consists of tiny particles (too small to see) that move around in all directions. To see what effect the loudspeaker has on these air particles, select the Particles option in the simulator. The simulator should now show tiny blue spheres that represent air particles. To get a clearer view also click the green + button to expand the window. Some particles are marked with a red X to help you focus on one at a time. Watch some of the marked air particles that are on the right side of the window, farthest away from the loudspeaker. As the sound waves move do these air particles move mostly forward and backward (in the same the direction that the waves are moving), or do they mostly upward and downward (perpendicular to the direction that the waves are moving)? Does the motion of the air particles suggest the sound waves are transverse or longitudinal? Does this agree with your conclusion from watching the motion of the loudspeaker in STEP 2? When the loudspeaker moves forward, it pushes the air particles directly in front of it forward also. As these air particles move forward they collide with other particles that are in front of them. Describe what happens to both air particles during such a collision. Explain how a series of such collisions leads to a longitudinal pulse moving through the air. WS-47

52 Unit WS STEP 4: We can describe the different regions of the longitudinal sound waves as they move through the air in terms of air pressure. As the loudspeaker cone moves forward the air particles in front of it have a smaller space to move around in. This increases the air pressure in that area slightly above its normal value. However, when the loudspeaker moves backward there is now a larger space for the air particles in front of it to move around in. Because of this, they now spread out more, decreasing the air pressure in that area slightly below its normal level. In the simulator these regions of low and high pressure can be seen as regions in which the particles are further apart and closer together than normal. Thus, as the loudspeaker (or other source of sound) vibrates back and forth the air pressure in front of it alternates between being slightly higher and slightly lower than normal. The collisions between the air particles are then the mechanism by which these high pressure and low-pressure regions move through the air, creating a sound wave. Note that it is disturbances in pressure that move out from the loudspeaker, not the air particles themselves. Looking at individual particles in the simulator shows that they simply oscillate backward and forward, becoming part of the successive regions of high pressure and low pressure that pass. Looking at the entire collection of air particles in the simulator, the regions of compressed particles can be seen as (partially) circular wavefronts that move outward from the loudspeaker. This means we can also regard a sound wave as a compression wave (the particles move such that successive regions of compression and rarefaction are created) or as a density wave (the particles move such that successive regions of high and low density are created). WS-48

53 Activity 4: Sound Waves When regarding sound waves in this way you can see that the behavior of the air particles is very like the behavior of the coils on a long spring that become stretched and compressed as a longitudinal wave moves along it. STEP 5: In reality it is impossible to see individual air particles, so return the simulator to the Grayscale representation and shrink the window by clicking on the red button. To find out how the shading represents high and low pressure, click the Show Graph button at the bottom of the window. A Pressure vs. position graph will appear that shows the air pressure along the dashed line in the middle of the window. (Note: While the pressure graph looks like a transverse wave it is important to note that the pressure wave itself is longitudinal. The graph is simply showing how the pressure caused by the wave is varying at different locations.) Is it the darker or lighter shading that represents a region of higher pressure? If you are not sure, pause the simulator and compare the graph and the grayscale display. Recall that the wavelength of a transverse wave is defined as the distance between successive peaks or successive troughs on the wave. Since sound waves are longitudinal in nature their wavelength is the distance between successive regions of high pressure or successive regions of low pressure. Below is a small section of a sound wave taken from the simulator. Mark a single wavelength (λ) in two different places on this picture. Keep the simulator window open as it will be needed in Exploration #2. WS-49

54 Unit WS Exploration #2: What are some properties of sound waves and how are they detected? You will need:! Large board! Computer with internet connection STEP 1: Now that you have seen how sound waves are created and how they move through the air, we will move on to investigate how they are detected and what some of their properties are. One of your group should hold a single sheet of paper at one end so it hangs straight down. Another group member should hold the large board up vertically about 2 feet from the paper and facing toward it. Now move the board forward and backward quickly (toward and away from the paper) many times while the other group members watch the paper. How does the paper behave while the board is being moved forward and backward? Why do you think the paper is behaving in this way? Explain your thinking in terms of what effect the moving board is having on the air in front of it, and what effect that air is having on the paper. The person with the board should now gradually move further away from the paper, still moving the board forward and backward as before. How does the movement of the paper change as the moving board gets further away? Why do you think this is? Just like the paper, the varying air pressure of sound waves can make any object move, even if only a little. WS-50

55 Activity 4: Sound Waves Describe an example from your everyday experience in which a sound wave has made a physical object move. The moving board and paper can also serve as an aid to understand the process of a person hearing a sound. How do you think the board and paper are like the process of human hearing? What do you think the board represents? What about the paper? As you are probably aware, the human ear contains a thin film, called the eardrum. When a sound wave enters the ear, the alternating regions of higher and lower air pressure make the eardrum move in and out, just like the paper was moved by the pulses in the air created by the movement of the board. A complex mechanism in the inner ear then converts these movements of the eardrum into electrical signals that travel to the brain, resulting in hearing 1. STEP 2: In an earlier unit, when examining the function of an electric buzzer in terms of ideas about energy, we simply said there was a sound interaction between the buzzer and some sound receivers during which energy was transferred between them. Here is a G/R energy diagram for a battery-powered buzzer (in an equilibrium state) as we might have drawn it earlier in the course. Electric Circuit Interaction Sound Interaction Energy Giver Energy Receiver / Giver Battery Energy Buzzer Energy Decrease in CPE Energy Receiver Sound Receivers Increase in energy 1 This is also how microphones work; the varying air pressure of a sound wave makes a small object move and this motion is turned into electrical signals WS-51

56 Unit WS However, now that we have investigated how sound waves are created, transmitted, and detected, we can expand this diagram to include this information. What type of interaction is occurring between the buzzer and the air when this is happening? (What do we call an interaction in which one object pushes on another with which it is in contact?) What type of interaction is occurring between the air and the eardrum of a listener? Complete the expanded G/R energy diagram for someone listening to an electrical buzzer? Check your diagram with another group. Electric Circuit Interaction Interaction Interaction Energy Giver Energy Receiver / Giver Energy Receiver / Giver Energy Receiver Battery Energy Buzzer Energy Energy Ear Drum Decrease in CPE Increase in STEP 3: From your real world experience you know that if a sound is very loud, one way to decrease how loud you perceive it to be is to move further away from its source. Bearing in mind how human hearing works, explain why you think this is in terms of the pressure variations in a sound wave as they move further away from a source of sound. WS-52

57 Activity 4: Sound Waves Return to the simulator and look at the Pressure vs. position graph. What happens to the amplitude of the pressure variations in the sound wave as it moves further from the source? Do they get bigger or smaller? How does this behavior help to explain why, as a listener moves away from a source of sound, the volume (loudness) she perceives gets less and less. STEP 4: Recall from previous activities that it is the motion of the source that determines the amplitude and frequency of a wave, but it is the properties of the medium it is moving through that determines its speed. You also saw that the wavelength of a wave is related to its frequency and the speed with which it moves by a simple relationship.!"#$%$&'(h!!"!!"#$ =!"##$!!"!!"#$!"#$%#&'(!!"!!"#$ We will now investigate how these ideas apply to sound waves. Suppose you changed the amplitude of the loudspeaker s back and forth motion, what do you think would be different (if anything) about the pressure variations in the sound wave it produces? Why do you think so? To check your thinking return to the simulator and vary the amplitude control. As the amplitude of the loudspeaker movement increases, does the amplitude of the pressure variations in the sound wave increase, decrease, or stay the same? WS-53

58 Unit WS Now suppose you were to increase the frequency of the loudspeaker s back and forth motion. Do you think this would make the sound wave move through the air faster, slower, or would it not affect the speed? Why do you think so? Do you think increasing the frequency would give the sound wave a longer or shorter wavelength, or would it not affect the wavelength? Why do you think so? STEP 5: To check your thinking about the speed of the sound wave when the frequency is changed return to the simulator and display the Stopwatch tool. Now watch the pressure graph as peaks and valleys in the pressure are generated at around the 10 cm position. These peaks and valleys then move to the right as the pressure wave moves trough the air. To get a measure of the speed of these waves, start the stopwatch when the pressure at the 10 cm mark reaches its maximum value and stop it when the peak that this creates reaches the end of the graph window. Repeat this measure for different settings of the Frequency slider control, but do not set it very high or it will be difficult to follow a particular peak. (Remember, you are looking at how fast a peak in the pressure wave moves from left to right, not how fast the graph goes up and down at one particular location.) WS-54

59 Activity 4: Sound Waves As you increase the frequency of the loudspeaker movement does the speed of the sound wave increase, decrease, or does it remain about the same? What evidence supports your answer? Since you know that it is only the properties of the medium that determine the speed of a wave, why does it make sense that sound waves of different frequencies all move through the air at the same speed? Now look at the distance between successive peaks in the wave as you vary the frequency. (You can either look at the grayscale display or the pressure graph.) As you increase the frequency of the loudspeaker movement does the wavelength of the sound wave get longer, shorter, or does it remain about the same? How do you know? Why does this relationship between the frequency of the loudspeaker movement and the wavelength of the sound wave make sense? (Hint: think about how far one region of high pressure will have moved through the air by the time the next region of high pressure is created by the loudspeaker.) WS-55

60 Unit WS STEP 6: Finally, let us investigate what effect changing some properties of a sound wave has on how the sound is perceived by a listener. When the amplitude of a sound wave is increased how do you think a listener perceives the sound differently, if at all? What about if the frequency is increased? To check your ideas, your instructor will lead a demonstration using a tone generator. As the amplitude of the loudspeaker movement is increased, what happens to how listeners perceive the sound? What about when the frequency is increased? Recall that the frequency of a wave is measured in units of Hertz (Hz). For sound waves this is a measure of how many cycles of pressure variation occur in one second. The human ear can only detect sound waves with a certain range of frequencies. Typically the range runs from a low of around 15 Hz (15 cycles per second) to an upper limit somewhere between 10 and 20 khz (a khz is one thousand Hz, so 20 khz is 20,000 cycles per second). Your instructor will use a tone generator to demonstrate how the actual hearing range can vary among individuals 2. According to this demonstration what is your own individual range of hearing? WS-56 2 Since this demonstration is not done under controlled conditions it should not be taken as definitive or diagnostic.

61 Activity 4: Sound Waves Note that the range of human hearing tends to diminish with age, particularly at the upper end. Other animals have very different ranges, with dogs and cats generally able to hear sounds at higher frequencies than any humans. Some bats can even hear sounds up to frequencies of 200 khz! Sound waves in other media Because the mechanism involves neighboring particles pushing on each other, sound waves can actually move through any medium. The speed with which they do so depends on how quickly neighboring particles can affect each other. (Because particles are moving more quickly at a higher temperature this means the speed of sound also depends on the temperature of the medium.) Also, because the particles in a liquid and a solid are closer together than in a gas, sound waves actually move more quickly through liquids and solids than they do though air. This table gives the typical value of the speed of sound waves through some different materials. Material Speed (m/s) Air at 0 C 331 Air at 100 C 386 Water at 25 C 1,490 Aluminum 5,100 Iron 5,130 Granite 6,000 Summarizing Questions S1: In most science fiction movies set in space, weapons, explosions, and spacecraft engines can be heard. Is this realistic? Why or why not? S2: If a tree falls in the forest and no-one is there to hear it, does it still make a sound? Explain your reasoning. WS-57

62 Unit WS S3: You are at the back of a crowd enjoying a concert in the park. At some point a violin plays a high E (frequency = 2637 Hz) at the same moment that a bass plays a low E (41 Hz). Which note will you hear first, if either, and why? S4: A loudspeaker is placed so that it is in contact with side-by-side regions of air, water, and iron, as shown in the diagram below. The loudspeaker is connected to a tone generator that is turned on and plays a note with a frequency of 200 Hz. Air Water Iron a) In which material would the 200 Hz sound wave created reach the other end first. Which would be last? Briefly explain how you know. b) In which material would the 200 Hz sound wave have the longest wavelength? In which would it be shortest? Using the simulator Grayscale representation, shade in the boxes in the diagram above to show how the wavelength of the 200 Hz sound wave would be different in each of these materials. Explain your reasoning. WS-58

63 Next%Generation%Physical%Science% and%everyday%thinking%! Waves,%Sound%and% Light%Module% Unit!L! Light!and!Color!!!!!! Studio1style!Class!!!

64 Unit L: Light and Color Table of Contents Activity # Activity (A) Title Page A1 Pinholes and Shadows L-1 Ext A 1 Drawing Light Ray Diagrams for Pinholes and Shadows online A2 Reflection of Light L-11 Ext B Further Investigations of Mirror Reflections and Images online A3 Refraction of Light L-23 A4 Color L-37 A5 ED Engineering Design: Designing a Periscope (forthcoming Summer 2016) L-53 1 Extensions (Ext s) are online homework activities.

65 UNIT L Developing Ideas ACTIVITY 1: Pinholes and Shadows Purpose As you are probably aware, scientists believe that light travels in straight lines. Like other scientific ideas, the idea that light travels in straight lines is supported by experimental evidence. Although you cannot actually see light traveling, there are many examples of phenomena that can only be explained by assuming light travels in straight lines. You will explore two examples of such phenomena in this lesson involving pinholes and shadows. (Pinholes are tiny openings in an otherwise opaque card or thin sheet.) As you investigate these phenomena, you should think about how your observations lend support to the idea that light does indeed travel in straight lines. The key question for this activity is: How do pinholes and shadows provide evidence that light travels in straight lines? Collecting and Interpreting Evidence You will need:! 2 Mini- Maglite flashlights! Black card with tiny hole (pinhole) in its center! Small black card (with no hole)! White screen! Stands for cards and screen! Ruler or straight edge for each group member (you may have your own)! Showcase bulb and holder 2016 Next Gen PET L-1

66 Unit L Exploration #1: What happens when a pinhole is placed between a light source and screen? STEP 1. We first introduce a representation of light that we will use when drawing diagrams in this unit. Unscrew the Maglite cap and stare at the point source for a few seconds as you hold it close to your eye. Probably, you can feel something going on with your eye. When you look at a light source, Light Interaction light coming from the source must Energy giver Energy receiver enter your eye in order for you to Eye-brain Flashlight Energy system see it. As you will read about in Activity 4 of this unit, light is another type of wave and so carries energy from an energy giver (the light source) to an energy receiver (a light receiver of some sort) via a light interaction. Thus, for the Decrease in CPE Increase in energy particular case of you looking at the flashlight, an appropriate G/R energy diagram would look like that shown here. (We treat the flashlight as a system and assume the battery and bulb in it are both in their equilibrium states.) We can also represent how light behaves by drawing by what we will call a light ray diagram. This light ray diagram illustrates two ideas: (1) light from a point source travels outward in straight lines in all directions; and (2) for you to see the point source, at least some light traveling outward from it must enter the eye. Light rays are always drawn with an arrow showing the direction from the light source to the object that the light strikes or enters (e.g. an eye). Note that the diagram above also shows some other light rays not entering the eye to illustrate that the light from this source goes outward in all directions. L-2

67 Activity 1: Pinholes and Shadows Next, hold the Maglite in front of the white cardboard screen, so light from the source illuminates the entire screen. Below, complete the light ray diagram showing how light from the source reaches, and therefore illuminates, the entire screen. In constructing this drawing we assume you are looking at the Maglite and the screen from the side, hence we only show the edge of the screen. We call this a side view. This makes it easier to draw a two-dimensional diagram, rather than trying to show a three dimensions in a diagram. STEP 2. Look at the black card with pinhole and make sure the hole is opened. If not, use a small object to make it a clear hole, but not too large. Place the black card with pinhole in between the tip of the light source and the screen. Make sure you observe a spot of illumination on the screen. Move the light source upwards, then downwards, then to the left and to the right, keeping it the same distance from the black card. Describe what happens to the spot of illumination on the screen when the flashlight is moved. Explain why you think the spot of illumination on the screen behaves this way. L-3

68 Unit L Now support your explanation by drawing a light ray diagram. Below we show a side view diagram of the light source, the black card with pinhole in its center and the screen. To the right of the diagram is a front view of the screen (looking directly at it). On this diagram: Use a straight edge to draw a light ray showing how light goes from the source, through the pinhole, and to the side view of the screen. (Your light ray should end at the side view of the screen.) Also, draw at least two additional light rays leaving the light source and striking the card away from the opening, suggesting that they are blocked from reaching the screen. Finally, draw a spot of light on the front view of the screen to show where it actually appears. (Line it up with where your light ray hits the side view of the screen.) Check your diagram with at least two other groups and try to resolve any differences. L-4

69 Activity 1: Pinholes and Shadows STEP 3. Imagine that you held two Maglites, one above the other, as suggested in the picture below. (Don t do it yet!) What do you predict you would see on the screen now? (A single spot of light again, two distinctly separate spots a line of illumination, or something else?) Explain your reasoning. Try it! [If you don t already have a second Maglite, borrow one from another group.] Describe what you observe on the screen. Now suppose you were use a finger to cover the top Maglite. Do you think either of the spots of illumination on the screen would disappear and, if so, which one (top or bottom). Why do you think so? Now try it. Observe and record which of the two spots of illumination on the screen disappears. Is that what you predicted? L-5

70 Unit L Move the two Maglites closer together so they touch each other, and then move them further apart. Describe what happens to the spots of illumination on the screen in each case. Use a straight edge to draw a light ray diagram that shows how, and where, the two spots of illumination appear on the screen. Remember to also include at least some rays that are blocked from reaching the screen by the card. STEP 4. Now imagine that you lined up ten Maglites, one above the other, so that each was almost touching the next one. Below, draw a light ray diagram and sketch what you think you would observe on the screen. Explain your reasoning below the diagram. L-6

71 Activity 1: Pinholes and Shadows Finally, instead of using several Maglite point sources, suppose you used a showcase bulb, which has a long, straight, vertical filament. If you placed this in front of the pinhole, what do you think you would observe on the screen and why? (Some possibilities might be a bright solid circle of illumination, several small dots one above the other, a vertical line, or a square.) Draw a light ray diagram above to show your thinking. Now try it and describe what you observe on the screen. If this is not what you predicted discuss with some other groups how you can explain it. Exploration #2: How can we explain the shape and size of shadows? STEP 1. Hold the tip of the Maglite about 25 cm in front of the center of the screen. Mount the small black card (with no hole) so it is about half way between the Maglite and the screen. The center of the card should line up approximately with the tip of the Maglite. You should observe a shadow on the screen. (If possible, block the light coming from the Maglites of neighboring groups.) L-7

72 Unit L How does the shape of the shadow compare to the shape of the blocker card? Are the edges of the shadow reasonably sharp? Write a couple of sentences to explain how you think the shadow is formed. When drawing a side-view light ray diagram to explain how a shadow is formed, the most useful light rays to draw are the two rays that leave the source and travel in straight lines just past the upper and lower edges of the blocker and then go to the screen. These boundary rays show where the upper and lower edges of the shadow region are, and also enable you to determine both the size and position of the shadow region on the screen (assuming a square-shaped blocker). Drawing some other light rays also shows where else the light reaches the screen and where it does not. STEP 2. Explore what happens to the position of the shadow as you move the Maglite upwards, then downwards, keeping it the same distance from the blocker card. Summarize your observations below. L-8

73 Activity 1: Pinholes and Shadows Draw a light ray diagram, including the boundary rays, to show how and where the shadow is formed when the Maglite is in the position shown in the diagram below. (Assume the shadow is approximately square in shape.) STEP 3. Now explore what happens to the size of the shadow as you move the Maglite closer to, then further away, from the blocker card. Summarize your observations below. Draw a light ray diagram to show why the shadow is larger when the Maglite is closer to the blocker card. L-9

74 Unit L Summarizing Questions S1. This activity was intended to provide you with evidence to support the idea that light travels in straight lines. a) Consider one observation you made with pinholes. Explain how it supports the idea that light travels in straight lines. (You might consider what you might have seen if this were not true.) b) Consider one observation you made with shadows. Again, explain how it supports the idea that light travels in straight lines. S2. A student in another class claimed that if you could get the Maglite TM a very long distance away from the blocker card, then (assuming it could still be seen) the shadow on the screen would actually be smaller than the blocker card itself. Do you agree or disagree with this student? Explain your reasoning and draw a light ray diagram to show your thinking. L-10

75 UNIT L Developing Ideas ACTIVITY 2: Reflection of Light Purpose Most people realize that light is necessary to see things, like images in mirrors, and various kinds of objects. But how does that happen? In this activity you will investigate how light behaves when interacting with shiny objects like mirrors and non-shiny objects, like paper. You will also consider how you see your image in a plane mirror. The key questions are: Initial Ideas 1. How does light reflect from shiny and non-shiny surfaces? 2. How do you see your image in a flat mirror? You instructor will show you a black piece of paper with a mirror and piece of white card on it. Imagine that this is placed in the middle of a table and three people stand around the table as shown below. With the room lights turned off, one person holds a flashlight so that it illuminates both the white card and the mirror, as suggested in the picture. mirror white card 2016 Next Gen PET L-11

76 Unit L Which of the three people (if any) standing around the table would perceive the illuminated card as being white. Why do you think so? If you thought one or more of the people standing around the table would not see the card as white, what do you think it look like to them and why? You are likely familiar with the observation a mirror can dazzle you when a bright light shines on it. Which of the three people (if any) standing around the table do you think might be dazzled when looking at the mirror in this arrangement? Why do you think so? If you thought the mirror would not dazzle one or more of the people standing around the table, what do you think the mirror would look like to them and why? Participate in a whole class discussion and make a note of any predictions or reasoning that is different from your own. L-12

77 Activity 2: Reflection of Light You instructor will now lead the class in carrying out this experiment. Record the results in the table below. For simplicity, limit your entries to one of Dazzled, White, or Black. What each person observed Flashlight person Side person Opposite person Appearance of mirror Appearance of white card You will now perform some investigations to help understand these results. Collecting and Interpreting Evidence You will need! Mini-Maglite flashlight! Plane (flat) mirror! Piece of clay! Tape Exploration #1: What happens when light strikes a shiny surface? STEP 1. Suppose you had a small, flat mirror and covered up all of it except for a narrow strip. Imagine that you put a point source (tip of Maglite ) on the table as shown below in this top view diagram. L-13

78 Unit L From which of the positions shown (A, B, C, D, E) do you think you would be able to see the image of the light source when looking at the narrow strip of mirror? (Choose as many as you think apply.) Show your thinking by drawing light rays on the diagram. Now try it! Tape an index card to the mirror, leaving only a very thin strip of mirror exposed (about a ¼ inch or less) at the right end. Stand the mirror and card upright on the table (using the clay for support). Lay the Maglite on the table as shown in the picture to the right. Narrow strip of mirror Light source Now, bend down so your eye is at tabletop level and use one eye to look at the strip of mirror from the five positions suggested in the diagram on the previous page. From which of the positions can you see an image of the light source when looking at the strip of mirror. Why do you think this is? STEP 3. In order to see the image of an object in a mirror, at least some light coming from the object must reflect from the mirror and come into your eye. That raises the question, how does light reflect from a shiny surface like a mirror? Do you think there is any particular rule for how light reflects from a shiny surface and, if so, what do you think it is? L-14

79 Activity 2: Reflection of Light In a moment you will check your thinking by watching a movie of a simulation. The setup is shown below, in which a narrow beam of light (red line) from a projector is aimed toward a mirror and reflects from it. (The narrow beam effectively shows us how a single light ray behaves.) Before you watch the movie we need to define some terms. Normal line: This is a line drawn perpendicular (at right angles) to the mirror at the point where the light beam strikes the surface of the mirror. [It is the dashed line in the picture above, which is vertical only because the mirror surface is horizontal. In general, the direction of the normal line will depend on the orientation of the mirror surface.] Angle of Incidence: This is the angle between the incoming beam (often also called the incident beam) and the normal line. Angle of Reflection This is the angle between the normal line and the reflected light beam. Now watch UL-A2 Movie 1, which shows the angle of incidence of the incoming beam being changed. A protractor is included so that you can observe the angles of incidence and reflection, and see how they compare. For all cases, how does the angle of reflection compare to the angle of incidence? L-15

80 Unit L STEP 4. A statement of how light behaves when it strikes a shiny surface, like a mirror, is called the Law of Reflection. When drawing light ray diagrams involving a mirror, you should always be careful to draw the reflected light ray so that the angle of reflection is as close to being equal to the angle of incidence as you can. (Note that you may have already seen this behavior when examining the reflection of waves from a boundary in Unit WS. Since light is also a form of wave (as will be discussed later in this unit) it is no surprise that it should obey the same Law of Reflection as other waves.) If light from an object strikes a mirror and then enters your eye, you will see the image of the object in the mirror. When the light enters the eye, your eyebrain system determines where the reflected light seems to have come from, and it is in this direction that the eye sees the image of the object. To determine which direction this is, we extend a light ray entering the eye backwards behind the mirror. This backward extended line is called a sightline. For example, in the diagram to the right, many light rays leave the source, only one of which reflects from the small strip of mirror into the eye. To draw the sightline lay a straight edge along the single light ray that actually enters the eye. Now extend this same line backward, behind the mirror, by drawing a dashed line, as shown in the diagram. Because the light entering the eye comes from this direction, the eye-brain system sees the image of the light source somewhere along this dashed sightline. However, if your eye is positioned so that no light from the object reflecting from the mirror can enter your eye, then you will not see its image in the mirror. L-16

81 Activity 2: Reflection of Light Furthermore, if no light from any other nearby objects reflect off the mirror and enter your eye, the mirror surface should then appear very dark (strictly it should appear black) when you look at it. Draw light rays on this diagram to show why the opposite person is dazzled when looking at the mirror. Also draw a sightline to show in what direction this person sees the image of the flashlight. Assuming there are no other lights in the room, explain why the flashlight and side person see the mirror as being a black surface. Exploration #2: What happens when light strikes a non-shiny surface? STEP 1. You have now seen that when a beam of light strikes a shiny surface (such as mirror) it is reflected in one particular direction. However, most objects are not shiny. For example, consider a white card. What do think happens when a beam of light strikes a white card. Do you think it reflects in one particular direction (like it would from a mirror) or does something else happen and if so, what? L-17

82 Unit L STEP 2. Turn on the flashlight and stand it upright at one end of the paper. Hold the white card as shown in the picture, so the flashlight beam strikes the card. Move the card in and out of the beam of light and observe what happens on the sheet of paper. Is light being reflected from the card and, if so, is it only in one direction, or in many different directions at the same time? How do you know? Below is part of a light ray diagram representing light from a Maglite TM striking a white card. Draw one or more light rays being reflected from the card to show that it reflects in many different directions. Now draw some eyes at different positions on the flashlight side of the diagram and explain why they would all see the card as white, no matter where they were. L-18

83 Activity 2: Reflection of Light Writing explanations using the ray model of light Now that we have established some of the behaviors of light when it reflects from both shiny and non-shiny surfaces we will use the light ray model to explain some phenomena using these ideas. To construct a scientific explanation using the ray model you should first draw a light ray diagram of the situation showing all the relevant behaviors of some of the light rays involved, and then write a narrative, keeping in mind that both should satisfy the usual criteria: Your explanation should be well-constructed. Your light ray diagram should be clear and easy to read and your narrative should be well written and easy to follow. (In particular, normal lines and sightlines should be easily distinguishable from light rays.) Also, the diagram and narrative should be consistent with each other. Your explanation should be accurate. The diagram and narrative should use one or more relevant ideas about the behavior of light that are consistent with those we have established from the evidence gathered in class. (It is particularly important to use a straight edge when drawing light rays.) Your explanation should be well reasoned. Reasons should be given for why the light behaved as it did. (Currently this would be based around a relevant general statement about how light reflects from a particular surface and what happens to it after it does.) When you drive at night, your headlamps shine on the road in front of the car. If there are white lines on the road (and the road is dry) then as you sit in the car behind the headlamps you can see these white lines quite easily. Bear the above criteria in mind as you consider the following explanation of how you can see the white lines on the road surface when you drive at night. L-19

84 Unit L Describe the model using a diagram: Driverʼs eye Headlamp White asphalt Road ahead White marker on road Write the narrative: The driver can see a white marker on the road ahead because light from the headlamp goes to the marker. Since the marker is a white, non-shiny surface, the light from the headlamp reflects from it in all directions. Because the driver is looking at the marker where the light is reflecting, he can see the marker. Do you think this explanation is good or problematic? Why do you think so? If you think either the diagram or narrative is poor, make appropriate corrections. Summarizing Questions S1. On the next page are four possible light ray diagrams showing light going from a lamp to a white, non-shiny ceiling, to a mirror on the wall, and then to an observer. The observer can see an image of the ceiling in the mirror. a) Which light ray diagram best represents how light behaves in this situation? What is problematic about the other three light ray diagrams? L-20

85 Activity 2: Reflection of Light b) On the appropriate diagram, draw a sightline to show in which direction the person sees the image of the ceiling. L-21

86 Unit L S2. When driving at night after it rains, and the road is very wet, it is very hard to see the white markings on the road ahead of you. This is because they look almost black to you, just like the rest of the road surface. Write an explanation as to why this is. Describe the model using a diagram: Write the narrative: L-22

87 UNIT L Developing Ideas ACTIVITY 3: Refraction of Light Purpose If you completed Unit WS you may have seen that when waves cross (at an angle) into a region in which their speed is different, they undergo refraction (they change direction). Since light is a type of wave, and since the speed of light is different in different materials, it is not surprising that this phenomenon occurs for light also. The most common circumstance in which we encounter refraction of light is when it enters or leaves a transparent material (such as clear plastic, glass, or water) and we make use of it in such devices as eyeglasses and telescopes. In this lesson we will explore this phenomenon further so that we can trace the path of light rays as they cross a boundary between materials. The key question for this activity is: How can we describe the behavior of light when it enters and leaves transparent materials? Initial Ideas You will need:! Large cup! Pencil Fill the cup about ¾ full of water and place the pencil in it, as shown here. When viewed from above it should appear like the pencil is broken or bent at the point where it enters the surface of the water. This is because the part of the pencil in the water looks like it is in a different place than it actually is Next Gen PET L-23

88 Unit L Here is a side view diagram of someone s eye looking at the pencil from above. Draw a single light ray leaving the tip of the pencil and show how you think it travels through the water and then the air to reach the eye. Lay a ruler along light ray that you have drawn entering the eye and extend it back it back to draw a sightline that shows in what direction the eye sees the image of the pencil tip. Briefly explain your diagram, including how it shows that the viewer sees the image of the pencil tip in a different direction to its actual position. Draw both your diagram on a large presentation board and participate in a whole class discussion about them. Make a note of any ideas that are different from your own. Collecting and Interpreting Evidence You will need! Computer with internet connection L-24

89 Activity 3: Refraction of Light Exploration #1: What happens when light enters a transparent material? STEP 1. To explain the effect you have seen, you need to understand how light behaves when it travels from one transparent material to another. We will do this in two steps, for now focusing on what happens when light enters a transparent material from the air. Open UL-A3 - Sim, which shows a laser beam projector near the the top left of the window. (A laser gives a nice narrow beam that will enable us to investigate how the light behaves. Effectively it is like examining the behavior of a single light ray.) The top half of the window is set to represent air and the bottom half represents water. The boundary between the two materials runs horizontally across the window. Turn on the laser beam by pressing the red button. This should produce a narrow beam of light aimed toward the surface of the water. Notice that some of the light is reflected at the surface according to the Law of Reflection that you saw in the previous activity, and some of the light goes into the water. (This is why you can see reflections when looking at the surface of a lake or other body of water.) For now we will ignore the reflected light and consider only the light passing into the water. Look at the diagram on the next page. Notice that the light beam that passes into the water has changed direction slightly at the surface. This change in direction is called refraction. (This occurs because the light waves travel more slowly in water than they do in air.) To analyze the behavior of the light beam, as with reflection, we draw a dashed line (called the normal line) perpendicular to the surface at the point where the light beam hits it. Notice that this line extends both above and below the surface. We then define two angles with respect to this normal line: the angle of incidence and the angle of refraction. L-25

90 Unit L Angle of Incidence: This is the angle between the incoming beam (the incident beam) and the normal line. (From now on we will simply show this angle as i in our diagrams). Angle of Refraction: This is the angle between the normal line and the refracted light beam. Notice that this angle is inside the water. (From now on we will simply show this angle as r in our diagrams). As the light beam enters the water does it cross over to the other side of the normal line, or does it stay on the same side as it was before entering? Suppose the light beam did not change direction as it entered the water. Use a straight edge to draw a dotted line on the picture above to show the direction of the beam in the water if it did not change direction. Evidently the light beam does change direction as it enters the water. Compared to the straight through dotted line you just drew in the water, is the refracted beam bent slightly towards the normal line or slightly away from the normal line? (In other words, is the refracted beam closer to the normal line than the straight through dotted line, or is it further away?) On the drawing above, is the angle of refraction equal to, less than or greater than, the angle of incidence? (In other words, which is larger, angle i, or angle r?) L-26

91 Activity 3: Refraction of Light STEP 2. Drag the protractor tool from the toolbox in the simulator and place its center at the point where the light beam enters the water. Make sure you align the 90-degree labels on the protractor with the surface (as shown here) otherwise your angle measurements will not be accurate. With this arrangement you can read off the relevant angles at the points where the incoming beam and the refracted beam each cross the protractor scale. In the example shown here the angle of incidence (i) is about 45 and the angle of refraction (r) is close to 30. (As before, ignore the reflected beam that heads back into the air.) Now move the laser so that angle of incidence for the beam changes. As the angle of incidence is changed, does the light always bend towards the normal line, or does it sometimes bend toward it and sometimes away from it? As the angle of incidence is changed, is the angle of refraction always less than the angle of incidence? What happens to the light if the angle of incidence is 0 degrees; that is, if the light strikes the surface along the normal line (perpendicular to the surface)? There is a law, called Snell s Law of Refraction, which provides an exact mathematical relationship between the angle of refraction and the angle of incidence for light going between different transparent materials. However, L-27

92 Unit L that law is complex and not convenient for quickly sketching light ray diagrams as we want to be able to do. Therefore, instead of using Snell s Law, we will use the rule of thumb below. When a light ray passes from air into a transparent material at an angle of incidence of 0 (directly along the normal line, perpendicular to the surface) it does not change direction. For any other angle of incidence, the light ray crosses the normal line and bends slightly toward the normal line as it does so, thus making the angle of refraction slightly less than the angle of incidence. We will refer to this idea as the approximate law of refraction for light going from air into a transparent liquid or solid material (such as water, plastic, or glass). When drawing light-ray diagrams for such a situation the way you draw them should correspond to this rule of thumb. STEP 3. Let us now practice drawing such diagrams. First we note that though the simulator shows the surface between the air and the transparent material as horizontal, and thus the normal line as vertical (because that is perpendicular to horizontal), this is not always the case. The surface of the transparent material could be at any angle, but the normal line is always drawn perpendicular to that surface. If the surface is not horizontal, the normal line will not be vertical. When drawing a light-ray diagram for such a situation we suggest you use the following procedure to draw what happens to a light ray as it goes into the new material. (You may also find it useful to use different colors to represent the different type of lines you will draw.) (1) Draw a dashed normal line (a dashed line that is perpendicular to the surface) that passes through that point where the light ray strikes the surface, and extend it into the air and into the transparent material. [Using the corner of a card or sheet of paper provides an easy way of drawing a normal line.] The angle of incidence (i) is the angle between this normal line and the incoming light ray. normal line i incident light ray L-28

93 Activity 3: Refraction of Light (2) Lightly draw a dotted line extending the incident light ray straight into the transparent material. This shows the direction the light would have traveled if it did not change direction. (We will call this the straight through line.) i straight-through line (3) Draw the refracted light ray inside the transparent material so that it i) is on the other side of the normal line and ii) is bent slightly toward the normal line (compared to the straight through line) and so that it makes an angle with the normal line (the angle of refraction r) that is slightly smaller than the angle of incidence. (In other words, the refracted light ray should be drawn slightly closer to the normal line than the straight through line is.) Remember to include an arrow on the refracted light ray. i r refracted light ray Now try drawing two of your own. Check with another group and try to resolve any differences that seem to be significant. (Since we are using an approximation, small differences are acceptable.) incident light ray incident light ray L-29

94 Unit L Exploration #2: What happens when light leaves a transparent material? STEP 1. Return to the simulator and set the angle of incidence for the incoming beam to 30. (The protractor should still be in the window in the appropriate place.) Assuming the top area is still air and bottom is water, the angle of refraction should be about 20. If a light ray were to start in water and approach a boundary with air at an angle of incidence of 30, do you think it would change direction in the same manner as when it passes from air into water with that angle of incidence? Explain your thinking. To check your thinking, use the control panels in the simulator to switch the upper area to be water, and the lower area to be air. The laser should now be in water, with its light beam passing from water into air. (As before, some light is reflected back from the boundary, but that need not concern us for now.) Notice that, even though the laser and incoming beam are now in the water, the angle of incidence is still the angle between the incoming beam (which is now in water) and the normal line, as shown here. Similarly the angle of refraction is still the angle between the refracted beam (now in air) and the normal line. The angle of incidence should still be 30. What is the angle of refraction now? L-30

95 Activity 3: Refraction of Light Does the light beam change direction in the same way passing from water into air as it did when passing from air into water with the same angle of incidence? If not what is different? When going from a transparent liquid or solid (like water) into air, does the light beam seem to bend towards the normal line or away from the normal line? (Is the refracted beam again closer to the normal line than the incident beam, or is it now further away?) STEP 2. Return to the simulator and move the laser to check what happens when the angle of incidence is in the range from 0 degrees up to about 48 degrees. (You will investigate what happens when the angle of incidence is equal to or greater than 50 degrees later.) What happens to the light if the angle of incidence is 0 degrees; that is, if the light strikes the surface along the normal line? As the angle of incidence is increased from 0 degrees up to about 48 degrees, does the light always seem to bend away from the normal line or does it sometimes bend toward it? As the angle of incidence is increased from 0 degrees up to about 48 degrees, is the angle of refraction always greater than or always less than the angle of incidence? L-31

96 Unit L Although Snell s Law of Refraction will allow you to calculate exactly what happens for any angle of incidence, we will again use a simpler rule of thumb for light going from a transparent liquid or solid material into air. When a light ray passes from a transparent material into air at an angle of incidence of 0 (directly along the normal line, perpendicular to the surface) it does not change direction. For an angle of incidence between 0 and almost 50 the light ray crosses the normal line and bends slightly away from the normal line as it does so, thus making the angle of refraction slightly greater than the angle of incidence. STEP 3. When drawing light ray diagrams for such situations you should use the same procedure as was introduced in Exploration #1, with a slight (but important) change to step (3). (Note also that the incident ray is now inside the material.) (3) Draw the refracted light ray in the air so that it is bent slightly away from the normal line (compared to the straight through direction) and so that it makes an angle with the normal line (the angle normal line of refraction r) that is slightly greater than the angle of incidence. (In other words, the refracted light ray should be drawn slightly further away from the normal line than the straight through line is.) straight-through line refracted light ray incident light ray Below are two diagrams representing light traveling in a transparent material (shown as a gray box) and about to pass out into the air. Use the appropriate procedure to draw the refracted light ray Check with another group and try to resolve any differences that seem to be significant. incident light ray incident light ray L-32

97 Activity 3: Refraction of Light STEP 4. Let us now check what happens when a light beam in water hits the surface with an angle of incidence equal to or greater than 50 degrees. Return to the simulator and move the laser to check what happens when the angle of incidence is 50 degrees or more. Describe what happens to the light beam when the angle of incidence is 50 degrees or more? When light goes from a transparent liquid or solid material into air, there is a certain angle of incidence called the critical angle of incidence, beyond which none of the light is transmitted into the air, but instead all of the light is reflected back into the liquid or solid. [For light going from water into air, the critical angle is approximately 50 degrees. The actual value of the critical angle depends on the specific liquid or solid material and may be a little different from 50 degrees.] This phenomenon is called total internal reflection and many applications rely on it. For example, optical fibers are thin flexible glass rods used in communication. They are constructed such that when a light ray strikes the surface from inside it is totally reflected back into the fiber and none of the light is lost out from the surface. The sparkle of a cut diamond is another example. The sides of the diamond are cut at specific angles such that most of the light rays entering the front faces from the air are totally reflected at the rear and emerge again at the front. Thus, when viewed from certain angles the diamond acts more like a mirror and produces a dazzling sparkle effect. L-33

98 Unit L Summarizing Questions S1. Try this interesting experiment. Place a coin on the bottom of a nontransparent cup, on the far side away from you, as shown here. Position your eye (close the other one) so the coin is only just hidden from your view by the edge of the cup. [The edge of the cup blocks light leaving the coin that would normally travel in a straight line to your eye.] While you hold your head still, have someone else pour water into the cup. Even though neither you nor the coin changes position, suddenly the coin comes into view and you see it! Write an explanation for why you are able to see the coin when there is water almost filling the cup. [For simplicity you can ignore the light that must travel from some source to illuminate the coin. Just treat the coin a s point source of light.] Describe the model using a diagram: (Draw a single light ray that can travel from the coin to the water surface, and then to your eye. Then draw a (dashed) sight line corresponding to this light ray, showing the direction where the person sees the coin.) L-34

99 Activity 3: Refraction of Light Write the narrative: (Describe how the light ray you drew on the diagram behaves and why. Also explain why this means the coin appears to be in a different place to where it actually is.) S2. Your friend drops her (waterproof) phone in a shallow pond at night and cannot see it to retrieve it. She asks you to shine a narrow-beam flashlight into the water it so she can see where it is. After scanning around for a while you locate it and hold the flashlight steady while it s beam illuminates the phone. On this side view draw a light ray diagram, showing how light goes from your flashlight to the phone, reflects from it (assume it is non-shiny) and them to your friend s eye so that she can see it. (You may re-aim the flashlight in any direction you think is necessary.) L-35

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101 UNIT L Developing Ideas ACTIVITY 4: Color Purpose The normal human eye can perceive a whole rainbow of colors, yet the sun and most light sources we are familiar with in everyday life, produce light that seems to us to be white. Of course, we also see colored lights, for example, in fountains, traffic lights, at dances, in theaters, etc. Is colored light completely different from white light, or are they connected in some way? Many interesting color lighting effects, especially those in live theatrical productions, depend on the use of transparent colored plastic materials called gels. Stained glass windows use transparent colored glass to produce their visual effects. Most colored objects we see around us, however, are opaque, not transparent. How do we see them? In this lesson you will explore these questions. The key questions for this activity are: 1. How do we see colored lights? 2. How do we see colored objects? Initial Ideas Your instructor will project red and green colored lights on a white wall (or screen). Now suppose your instructor were to move the lights so that they partially overlap on the screen.. What color do you think you would see in the region where they overlap? Why do you think so? 2016 Next Gen PET L-37

102 Unit L! Participate in a class discussion and make a note of any ideas or reasoning that are different from yours. Following the discussion your instructor will overlap the red and green lights. What color do you see in the region of overlap? Is this what you predicted? Light as an Electromagnetic Wave As you may be aware, light is a type of wave. You may have studied some types of wave in a previous unit of this course. Those were called mechanical waves because they need a medium (some type of material, such as a string, water, or even the air) to move through. However, light waves do not need a medium to move through. When electric charges oscillate they create disturbances in the electric and magnetic fields around them. These fields exist throughout the whole universe within all materials, and even in the vacuum of space. The disturbances created by oscillating charges move away from the source (oscillating charge) as electromagnetic (EM) waves. Depending on the details of how the oscillating charges move, these EM waves can have any wavelength. Together, all the different types of EM waves form what we call the electromagnetic spectrum, with a continuous range of wavelengths from many meters (radio waves) to less than a trillionth of a meter (gamma-rays). What we call visible light is actually only a small section of the complete EM spectrum as shown here. L-38

103 Activity 4: Color Collecting and Interpreting Evidence You will need " Envelope with three color gels (red, green, blue) " Two flashlights with narrow beams " Tubular bulb in socket " Spectral glasses (one pair per person) " Computer with internet connection " Second envelope with three different color gels (yellow, cyan, magenta) Exploration #1: How do we see colored lights? STEP 1. Turn on your flashlights and (if possible) focus each of them so they give a small bright white spot when shone on a sheet of white paper. Now hold the red gel over one flashlight and the green gel over the other one, and shine them onto the paper from the same distance. What color do you see where the red and green lights overlap? (This should be close to the same as your instructor s demonstration.) What color do you think you would see if you overlapped red and blue spots of light? What about blue and green? Prediction: Red + Blue = Blue + Green = Now use the appropriate color gels to check your predictions. Do your observations match your predictions? If not, describe what colors you do see. L-39

104 Unit L STEP 2. We will now try to understand these results. Plug in and turn on the light bulb. It should look something close to white in color to you. Now look at it through your spectral glasses. You should see many bands of colors. The range of colors that you see is called the color spectrum of the white light source. (The glasses act a like prism to create a spectrum from the white light coming from the bulb.) Concentrate on the band that seems to spread out to the imediate left from the light source. You should see the familiar rainbow, the colors of which are sometimes simply referred to as ROYGBV. (Red, Orange, Yellow, Green, Blue, Violet) You are seeing this spectrum because the human eye is sensitive to the wavelengths of the electromagnetic spectrum between about 380 nm (perceived as violet light) and 730 nm (perceived as red light). (1 nm = 1 x 10-9 m.) A normal human eye has three different types of color sensitive receptors (called cones ) in it that are sensitive to different regions of this color spectrum. One type is sensitive to the red, orange, and yellow region of the spectrum. A second type is sensitive to the middle regions (yellow, green and a little of the blue region). The third type is sensitive to the blue and violet regions. Though there is some overlap of these regions of sensitivity, it is convenient to divide the entire color spectrum into three broad bands, which we will call red (R), green (G) and blue (B), as shown on the next page. L-40

105 Activity 4: Color STEP 3. Let us now try to understand how normal color vision works using these ideas and a simulator. Open UL-A4 - Sim where you will see a man facing red, green, and blue lights, each with dimmer controls attached. Slide the dimmer control on the red light up to its highest setting. (Note that, in this simulation, the light is represented as tiny particles, rather than as waves. Although we will not explore that model further, scientists often think of light as consisting of a stream of particles, called photons. In the simulation the particles are colored as red, green or blue. The coloring is only a simulator representation to help you keep track of which wavelength band the particles are from; in actual fact, the particles of light are not colored. ) There should currently be a stream of photons from the long wavelength (R) band of visible light entering the person s eyes. His (and hence our own) eye-brain system perceives this as red light because this wavelength band mostly triggers only the R receptors in our eyes. Similarly, light from the middle range of wavelengths triggers our G receptors and light from the shortest range of wavelengths triggers our B receptors. L-41

106 Unit L Suppose equal intensities of light from both the long wavelength band (R) and the middle wavelength band (G) simultaneously enter our eyes, what color do you think we would perceive and why? Check your thinking by setting both the red and green lights in the simulator to their highest setting. What color is perceived? Is this what you predicted? Next, check what color is perceived when equal intensities of red light and blue light enter the eye, and then when green light and blue light enter the eye. What color is perceived when equal intensities of red and blue lights enter the eye? What color is perceived when equal intensities of green and blue lights enter the eye? To ensure consistency we will use the name CYAN for the mixture of blue and green, and the name MAGENTA for the mixture of red and blue. Now predict what you think would be perceived when equal intensities of red, green and blue light enter the eye? Explain your thinking. Before you check with the simulator, borrow a flashlight from another group and try it yourself. L-42

107 Activity 4: Color What color do you see when red, green and blue are overlapped? Is this confirmed by the simulator? Is it what you predicted? Why does this result make sense in terms of which color receptors are being triggered in your eyes? We can summarize the simple rules for mixing equal intensities of color lights as follows. We will use the shorthand notation, R, G, B, W, Y, C and M to represent red, green, blue, white, cyan (blue-green) and magenta (red-blue) light respectively. R+G = Y R + B = M G + B = C R+G+B = W We can perceive other color lights (e.g. orange, lime green, etc.) by mixing different intensities (brightnesses) of R, G and B lights. Try it now with the simulator. This process is known as color addition. Finally, check what color is perceived when no light of any color enters the eye. Why does this make sense? Lots of devices rely on color addition to produce a seemingly wide range of colors from just red, green, and blue. If you look very closely (with a magnifier) at a computer monitor or a TV screen you will notice a large number of very tiny and closely spaced dots or stripes of red, green and blue. When viewed from far enough away, the visual effects of tiny, closely spaced dots or stripes are similar to overlapping color lights. Thus, if a region on the screen has brightly glowing red and green dots/stripes, but the blue dots/stripes are turned off, then beyond a certain distance away that part of the screen will appear to a viewer to be yellow in color. The Pointillist artists of the 19 th century, the most famous being Georges Seurat, used a similar technique with small, closely spaced dots of color paint. To see the effects they wanted, you need to stand far enough back from the painting so your eye no longer detects the individual dots, but instead sees the additive combination of the colored dots of paint. L-43

108 Unit L Exploration #2: How do color gels work? STEP 1. In Exploration #1 you produced colored light by placing a color gel over a flashlight, the light from which normally looks white. We will now consider how color gels work to make colored light from white light. Assuming the lights in your room are white, look at them through the red gel. (Do not use the spectral gasses.) They should appear red to you. Three students are discussing what the red gel is doing to the light. Which student do you agree with and why? L-44 STEP 2. To find out what a color gel does you will make some observations using the spectral glasses and a light bulb with a long straight filament. Each member of your group should make the following observation. Put on your spectral glasses and look at the glowing bulb. Focus on the color spectrum that appears to the left of the bulb filament itself. Close one eye and slowly move one of the gels (any color) in front of the other (open) eye.

109 Activity 4: Color To see what the gel does, you need to compare what the spectrum looks like both with and without the gel in front of your eye. To do this, you should hold the gel in a position such that it is covering only the bottom half of the full color spectrum you are seeing. (Alternatively you could move the gel completely in front of and then away from your open eye, several times.) Based on your observation, does the gel your are using seem to add its color to the light (in which case certain colors would be brighter when seen through the gel), take away some colors from the spectrum (certain colors would be dimmer or disappear completely), or both add and take away? STEP 3. Now you ll do a more careful set of observations. As you have now seen, the color gels seem to remove certain colors from the spectrum and we will now determine what these are for each color gel. To make the observations and analyses simpler, we will assume that the full color spectrum is made up of only the three broad bands mentioned earlier: red (R), green (G) and blue (B) and determine which of these three bands each gel removes from the white light and which it lets through. (When we say the a color gel removes one or more color bands from the light, what actually happens is that inside the gel there is a chemical dye that has the property of absorbing a particular band or bands of light.) For example, when you look at the spectrum through the red gel, you should see that the R band looks about the same both with and without the gel. This means that the R band is let through (transmitted) by the red gel. However, you should also see that, when viewed through the red gel, most of the B and G bands are missing (or at least are significantly dimmer). This means that the B and G bands are removed (absorbed) by the red gel. Note: These gels are not perfect and so in some cases you may see a little of the other bands, but at least some parts of one or more bands should be removed, or be somewhat dimmer than they are without the gel. L-45

110 Unit L Make observations with your spectral glasses and gels to complete the table below according to which of the R, G, and B bands each gel seems to remove (absorb) and which it lets through (transmitted). (Remember to make allowances for the fact that the gels are not perfect!) The first line has been done for you. Table 1: Which color bands of the spectrum do each of the gels seem to remove and which do they let through? Name of gel Which color band(s) are removed (absorbed)? Choose from R, G and/or B Which color band(s) are let through (transmitted)? Choose from R, G and/or B Red G B R Green Blue Yellow Cyan Magenta Recall from Exploration #1 that when red, green and blue light enter the eye it is seen as white light. So when we look at the white light source without the spectral glasses, we can imagine that all three of the red, green and blue bands of the spectrum are entering our eyes. Why do we see green when we look at a white light source through a green gel? Why do we see magenta when we look at a white light source through a magenta gel? L-46

111 Activity 4: Color STEP 4. Sometimes it is convenient to label a color gel by what components it removes from the white light. For example, an R gel could also be called a G B gel (minus G, minus B), and a magenta (M) gel is a G gel (minus green), and so forth. Another name for a color gel is a color filter, which already suggests that it removes something from the light. When we use gels or dyes to remove colors from light we call it color subtraction. We can add information about color to a light ray diagram to help us when working with the ideas of color subtraction. For example, a diagram for a person looking at a white light through a magenta gel would look like this. White light source RGB Magenta gel ( G) RB The light being emitted by the source is white, so it contains all three of our color bands (R, G, B). A magenta gel can be regarded as a G filter and so it removes the G band from the white light, leaving the R and B bands to enter the eye. As you saw in Exploration #1, we perceive this particular combination of bands as magenta. To check your understanding of these ideas consider the following questions. Be sure to make your predictions before trying the experiments! Suppose you overlapped a cyan (C) gel and a green (G) gel and looked at the white room lights through them both. (NOT using the spectral glasses!) Predict what color you would see. To help, draw a color light ray diagram that shows white light passing first through a cyan gel and then through a green gel, before the light enters your eye. L-47

112 Unit L Would it matter whether the white light passed through the cyan or the green gel first? (If you are not sure, draw another diagram with the gels switched in the order the light passes through them.) Try it now. What color do you see when looking at the white room lights through overlapping cyan and green gels? (Do not use the spectral glasses!) If this is not what you predicted try to explain it. Now suppose you looked at the room lights through a combination of overlapping yellow and magenta gels. Draw a color light ray diagram to help you predict what color you would you expect to see. Try it now. What color do you see? If this is not what you predicted try to explain it. Finally suppose you looked at the room lights through a combination of yellow, cyan and magenta gels. What color (if any) would you expect to see and why? Try it now. What color do you see? If this is not what you predicted try to explain it. L-48

113 Activity 4: Color Exploration #3: How do we see colored objects? STEP 1. Consider looking at a piece of red paper being illuminated by a white light source. Why do you think the paper looks red to you? STEP 2. Before we explore how we see colored objects, let us first remind ourselves what happens when light hits a non-shiny object. Turn on the flashlight and stand it upright at one end of the paper. Hold the white card as shown in the picture, so the flashlight beam strikes the card. Move the card in and out of the beam of light and observe what happens on the sheet of paper. Is light being reflected from the card and, if so, what color is that light? How do you know? The light hitting the card (from the flashlight) is white, which we can regard as a combination of the R, G, and B color bands. What color bands are being reflected from the white card? How do you know? Complete this color light ray diagram to show your thinking. (Be sure to show in what direction(s) you think the light is reflecting and what color bands are in the reflected light.) White card RGB White light source L-49

114 Unit L Now repeat the experiment using the red card instead of the white card. While looking at the white paper, move the red card in and out of the flashlight beam. Is light being reflected from the card and, if so, what color is that light? How do you know? What color bands are being reflected from the red card and which are being absorbed? How do you know? Complete the color light ray diagram to show your thinking. Red card RGB White light source As you have probably deduced, the dye in the red card absorbs the green and blue bands of light and reflect the red band. In this sense the way light is reflected or absorbed by a colored object is the same as the way it is transmitted or absorbed by a colored gel. Use these ideas to explain why a yellow piece of paper appears yellow when illuminated by a white light source. L-50