IDS 102: Electromagnetic Radiation and the Nature of Light

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Clara Terry
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1 IDS 102: Electromagnetic Radiation and the Nature of Light Major concepts we will cover in this module are: electromagnetic spectrum; wave intensity vs. wavelength, and the difference between light reflection, transmission, and absorption. WAVES, LIGHT, and the ELECTROMAGNETIC SPECTRUM What is a wave? For our purposes, we will think of a wave as something that travels from one place to another. The shape of a wave is usually something like alternating bumps and valleys: first a bump, then a valley, then a bump, and so on. Here is a cartoon of a wave going by on the surface of a body of water, moving to the right. What am I doing here? I'm a hummingbird! As you may have noticed, there is a hummingbird hovering just above the surface of the water in the middle of one of the "valleys" (between two bumps). If our hummingbird continues to hover in that specific place in space (so that it does not move), what will happen to it as the wave moves? (Think about it! This may seem obvious, but it is an important point about the behavior of waves.) You probably concluded that the hummingbird would get wet. Now for the important point about waves Two students are arguing. Student #1 says that waves move back and forth in a zigzag type motion. Student #2 says that unless something gets in the way waves move (for the most part) in straight lines. "The shape of the wave," Student #2 says, "is not the same as the direction that the wave is going." Based on your ideas about the wave above, which student do you and your hummingbird agree with? Did that wave move in a straight line or in a zigzag? Check with your instructor before continuing 1

2 Wavelength and the Energy of waves: Among the other things that waves do, they carry energy. Our poor hummingbird is smacked with the energy of a passing water wave. Every time a wave hits the hummingbird, it was to use more energy to fly forward. Keeping this in mind, consider the 2 situations below: Wave #1 Wave #2 Remembering what you know about the meaning of the word intensity, in which situation does the wave have a higher intensity? Water isn t the only substance that transmits waves of energy. ( Transmission happens when a wave passes through a substance. For example, air transmits both light and sound waves. Glass transmits light waves well, but it does not transmit sound waves well.) Describe at least 2 examples of the English words used to describe the intensity of different types of waves. Wave energy can also vary when waves are the same height. Consider the situations below: Wave #1 Wave #2 For which situation would you say that the waves of water have more energy. Explain your reasoning. Summary: Hopefully by now you have concluded that waves travel in straight lines, not zigzags, and that waves in which the crests are close together seem to carry more energy than long waves. If you are having trouble believing either of these things, you are not alone. They are two of the most important and most misunderstood properties of waves. 2

3 Light Waves: Light travels in waves. Remember, this does not mean that light travels along a zigzag path. It means that light travels in "packages" of energy. We usually think of a wave as something that goes by in lumps and bumps. First one bump goes by, then another, and then another. We don't notice light going by that way because the bumps are so small and they go by so fast. When a wave of orange light reaches our eyes, for example, there are half a million bumps crammed into every foot, and it only takes a nanosecond (one billionth of a second) for those half a million waves to go by. Still, it is precisely the size of those waves, so small that we can fit half a million of them into one foot (or more precisely 1.5 million into each meter) that tells our eyes that we are looking at orange light and not blue light (which has 2 million waves per meter) or red light (which has 1.3 million waves per meter). We call the length of a wave the wavelength. Which one has longer waves, the blue light or the red light? Explain your reasoning. Which one do you think carries more energy, the blue light or the red light? Explain your reasoning. It turns out that the blue light and the red light move with the same speed (three hundred million meters per second). We call the number of waves that pass each second the frequency. Which one sends more waves past your eye per second (a.k.a. which has a higher frequency)? Explain your reasoning. Summary: As the wavelength of a wave increases, the frequency of the wave and the energy of the wave. 3

4 The visible light spectrum When we compare blue light to red light we see that blue light has a shorter wavelength, higher frequency, and carries more energy for the same amount of brightness (red light has a Longer wavelength, Lower frequency, and Less energy the L s go together). Still, what s the fun of knowing that if you don t understand color? It turns out that most of us have eyes that detect three colors of light: Red, Green, and Blue. Some people detect fewer colors (they have partial color blindness) but nobody detects more. * Every other color you have perceived in your life has been a mixture of those three colors of light. Every color on a computer monitor is a combination of red, green, and blue dots. ACTIVITY #1: Open Microsoft Word to a new document page and look at the white page on the screen with a magnifier. Describe what you see under the magnifier: ACTIVITY #2: Now pick up a diffraction gradient or a pair of prism glasses. Examine the light from one of the incandescent light bulbs and one of the light emitting diodes, or LEDs. Describe what you see for each light source (don t turn the lights off when you re done). Now carefully test the temperature of each light. Don t touch the incandescent bulb, but describe what you feel when you placing your hand near the bulb and how this differs for the LED. 4

5 An incandescent light bulb is designed to create or Emit light when an electrical current is run through a thin wire or filament. In other words, the light bulb is an apparatus that changes electrical energy into light. Unfortunately traditional incandescent light bulbs are not very efficient; only ~10% of the electrical energy they consume is turned into light, most of the rest is lost as unwanted heat (remember that heat is also a form of energy). LED lights work on a different principle so that only 80-89% of the energy they produce is turned into visible light. There is a whole spectrum of colors. The spectrum is the complete collection of all possible wavelengths. When we separate all of the different wavelengths that are hidden in white light, we see the spectrum as a rainbow. Some additional concepts: Absorption, reflection, and transmission When light encounters a substance, there are three things that can happen, and sometimes they all happen at once. 1. The light can be reflected which means that it bounces off. It changes direction, but aside from that it is pretty much unchanged. A mirror is very smooth and it reflects light all in the same direction. A piece of sandpaper is rough and it scatters light in all directions. Most objects are somewhere in between. Most of the objects we see in our everyday world reflect light to our eyes that is why we see the objects. (Some people have the misconception that in a totally dark room, your eyes will eventually adjust so that you can see objects in the room. This is not true! If there is no light to reflect off an object, we would not see the object!) 2. Light can be absorbed which means that the energy in the light is absorbed by the substance. Something that absorbs some colors (or wavelengths) of visible light is called a pigment and it is what we use to make paint. When light is absorbed, the light is gone but the energy remains in the substance in another form. (Hint of things to come: the energy usually comes back out!) 3. Light can be transmitted which means that it passes through the substance. A window is clear because visible light is transmitted. Stained glass appears brightly colored because some colors (or wavelengths) are absorbed and others are transmitted. Something that transmits some wavelengths but not others is called a filter. A substance that absorbs some colors and reflects others is called a pigment. We say that the three primary colors of pigment (or paint) are yellow, cyan, and magenta. (In primary school you probably learned that the primary colors of paint were red, blue, and green, but you never could get that cool magenta or turquoise color, could you?) 5

6 ACTIVITY #3: Find some colored paper (pigments) and filters (translucent plastic). You should have magenta, yellow, and cyan sheets of paper and at least a red and blue filter. (Our cyan paper is not truly cyan, but it is close!.) Hold up your paper so that white light is hitting each of the colors of paper. How will the appearance of these papers change when you use a red or a blue filter. (Remember, the red filter only lets red light through and the blue filter only lets blue light through). Make a prediction: Cyan Magenta Yellow Color under red filter Color under blue filter Place the three sheets of paper so that they are overlapping but you can see all of them. Place the red filter over them and record your observations. Place the three sheets of paper so that they are overlapping but you can see all of them. Place the blue filter over them and record your observations. Now describe what you observed above using the concepts of emission, absorption, transmission, and reflection in the sentence below. The light bulb white light. The red filter all colors except red and it red light. Under white light, the red paper red light. The blue filter all colors except blue and it blue light. Under white light, the blue paper (such as red)/ blue light while it absorbs other colors 6

7 ACTIVITY #4: Some other light sources (fluorescent bulbs, neon lights, sodium lights) produce light through specific atomic changes. Look at a neon light, sodium light, or other chemical gas light through the rainbow glasses provided. What do you notice about the spectrum produced by neon light, sodium light, or other chemical gas light? How would you describe the difference between the spectrums produced by the incandescent bulb and the chemical gas bulb? The spectrum of the incandescent bulb appears to be more more what? Think about rainbows that you have seen in the sky. These are the spectrum of the Sun. What does this tell you about the spectrum of the Sun? Is the spectrum more similar to the incandescent bulb or the gas bulb? We are progressing toward an understanding of the Earth s greenhouse effect. The process involves the transmission, absorption and emission of energy in the form of waves. Please let us know if you do not understand these properties ( 7

8 Electromagnetic waves Most of the waves we are familiar with, such as waves in water and sound waves, require a medium (or substance) to travel through. However, light is part of a spectrum of electromagnetic radiation that will travel through a vacuum (no substance). Electric and magnetic "fields" can carry waves the same way the surface of a body of water can. Light travels along as "bumps and valleys" of electrical "pushes and pulls." Honest. When this property of light was discovered, it immediately raised a question. We see electromagnetic waves (EM waves) with wavelengths between 450 nanometers (blue) and 700 nanometers (red). We call them light. Are there EM waves with longer wavelengths? Shorter wavelengths? The physics of electricity suggested that there would be, but we could not see them. We now know that there are EM waves with wavelengths thousands of times shorter than blue light (and thus energy thousands of times greater than light). There are EM waves with wavelengths longer than light, too. Our eyes don't detect them, but they are important in nature and we use them in technology. ACTIVITY #5: Using the EM chart above rank order the list of wave types in order (1 means largest wavelength) from those types that have the longest wavelengths to those that have the shortest wavelengths. Then rank the types of energy according to the amount of energy carried by each type of wave in the space below (1 means the greatest energy) Type of radiation Wavelength Energy Gamma waves Infrared x-rays Radiowaves Ultra violet Visible Light Recall that waves that have a short wavelength have the highest energy. This is the reason that there are limits to amount of x-rays a person should be exposed to during a certain period of time (this is mostly an issue for x-ray technicians rather than the patients). Fortunately for us, the Sun does not produce a lot of gamma rays and x-rays. Most of the gamma rays and x-rays that come to the Earth from elsewhere in the universe are absorbed in the far upper atmosphere (above the troposphere). The small amount of high energy EM radiation reaching the Earth is a good thing because otherwise life on Earth as we know it would not be possible. A short Review: As electromagnetic radiation from the Sun arrives at the Earth. On a separate piece of paper describe what are the three things that can happen to this energy? 8

Chapter 17, Electromagnetic Waves Physical Science, McDougal-Littell, 2008

Chapter 17, Electromagnetic Waves Physical Science, McDougal-Littell, 2008 SECTION 1 (PP. 553-558): ELECTROMAGNETIC WAVES HAVE UNIQUE TRAITS. Georgia Standards: S8P4a Identify the characteristics of electromagnetic and mechanical waves; S8P4d Describe how the behavior of waves