Chapter 13 Light and Reflection

Transcription

1 Chapter 13 Light and Reflection

2 It is likely that some of you are going to find this chapter fairly difficult. My advice: Do not fall behind Complete the practice problems in class while I am completing them. Even the ones I do on the board. If you need help, stay in office hours (Tuesday and Thursday after school)

3 Chapter 13-1: Characteristics of Light Pages 442 to 446 By the end of this section you should be able to: q Identify the components of the electromagnetic spectrum q Calculate the frequency or wavelength of electromagnetic radiation q Recognize that light has a finite speed q Describe how the brightness of a light source is affected by distance.

4 Electromagnetic (EM) Waves - EM waves are non-mechanical waves, meaning they do not need a medium to travel through. - They contain both an electric and a magnetic field. - Formal Definition: Waves which are synchronized oscillations of electric and magnetic fields that propagate at the speed of light through a vacuum.

5 Electromagnetic Spectrum

6 Prisms can separate white light

7 Name of the band? Name of the album? Year of the album? How many weeks did this album stay on the Billboard Top 200?

8 Name of the band? Pink Floyd Name of the album? Dark Side of the Moon Year of the album? 1973 How many weeks did this album stay on the Billboard Top 200? 741 weeks, (from )

9 Do not call out answers! We will discuss after. 1. How many different types of electromagnetic radiation (EMR) are there? 2. Which type of EMR has the shortest wavelength? 3. Which type of EMR has the highest frequency? 4. Which color of visible light has the highest frequency? 5. Which color of visible light has the longest wavelength?

10 Do not call out answers! We will discuss after. 1. How many different types of electromagnetic radiation (EMR) are there? 7 types 2. Which type of EMR has the shortest wavelength? Gamma rays 3. Which type of EMR has the highest frequency? Gamma rays 4. Which color of visible light has the highest frequency? Violet 5. Which color of visible light has the longest wavelength? Red

11 Light travels slower in a medium than in a vacuum Speed of light in vacuum: x10 8 m/s. Speed of light in air: x10 8 m/s. Speed of light in ice: x10 8 m/s Speed of light in water: x10 8 m/s Each time light hits an atom, it is absorbed and then re-emitted. Since this process takes time, it slows down the process. The more dense the medium, the slower light travels ** Recall that ice is actually less dense than liquid water, hence why ice floats in a glass of water.

12 When am I ever going to need science? I want to be a news reporter

13 The sun is 149,600,000 km from the Earth! That s the length of 1,496,000 soccer fields! Question: Assuming the circumference of the Earth is km, how many laps around the Earth could light make in 1.0 second. Recall, the speed of light in air is x10 8 m/s. Note: You will have to do a unit conversion.

14 Solution Question: Assuming the circumference of the Earth is km, how many laps around the Earth could light make in 1.0 second. Recall, the speed of light in air is x10 8 m/s. Note: You will have to do a unit conversion. Convert the circumference to meters: km x (1000m/1km) = 40,075,000 m Find out how far light travels in 1.0 second (v = d/t) d = vt = (2.9970x10 8 m/s)(1.0 s) = x10 8 m/s Divide your answer by the circumference of the Earth to find out how many laps it would do: Laps = (2.9970x10 8 m/s)/(4.0075x10 7 m) = laps = 7.5 laps

15 The speed of light who figured this out? Galileo Galilei ( ) first tried to determine the speed of light, but was unsuccessful. When watching a canon be fired off in the distance, it was observed that the flash of light could be heard before the sound. He concluded that light travels faster than sound. Ole Römer (Danish astronomer) in 1676 made the first real (although slightly incorrect) measurement of the speed of light at an observatory in Paris. Using eclipses on Jupiter created by its moon Io, he was able to determine the speed of light to three-quarters of the real value. If you want to read more, check out the following link:

16 All electromagnetic waves move at the speed of light Note: For all calculations in this chapter regarding the speed of light, we will be using 3.00x10 8 m/s. Wave speed equation: c = f λ c = speed of light (m/s) = 3.00x10 8 m/s f = frequency (Hz) λ = wavelength (m)

17 Example Problem The most common FM radio bands used for broadcasting radio is 87.5 to 108 MHz. What are the longest and shortest wavelengths in this frequency range?

18 Solution Convert MHz to Hz to be able to use the wave speed equation: 87.5 MHz * (1,000,000Hz/MHz) = 87,500,000 Hz 108 MHz * (1,000,000 Hz/MHz) = 108,000,000 Hz Calculate each individual wavelength For 87.5 MHz: c = f λ so 3.00x10 8 m/s = (87,500,000 Hz) λ λ = m = 3.43 m For 108 MHz: c = f λ so 3.00x10 8 m/s = (108,000,000 Hz) λ λ = m = 2.78 m The range of wavelengths is from 2.78 m to 3.43 m.

19 Luminous Flux Luminous: Full of or shedding light; bright or shining Flux: Continuous change Luminous Flux: The rate of which light is emitted from a source. Luminous flux is measured in lumens (lm). The luminous flux divided by the area of the surface is called the illuminance (measured in lm/m 2 ), decreases as the radius squared when you move away from a light source.

20 Chapter 13-2: Flat Mirrors

21 Chapter 13-2: Flat Mirrors Pages 447 to 450 By the end of this section you should be able to: q Distinguish between specular and diffuse reflection q Apply the law of reflection for flat mirrors q Describe the nature of images formed by flat mirrors.

22 How light is reflected depends on the surface

23 Incoming and Reflecting Angles are Equal

24 Reflection of flat mirrors The simplest mirror to analyze is the flat mirror (versus concave/convex) Flat mirrors create virtual images: An image from which light rays appear to diverge, even though they are not actually focused there. A virtual image cannot be projected on a screen. Appears to be behind the mirror - virtual

25 Practical Uses of Flat Mirrors Interior Decorating The use of mirrors can make hallways look longer or rooms look larger. Small clothing stores often use this technique to look larger.

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28 Practical Uses of Flat Mirrors Interior Decorating Double the amount of sunlight in your room by allowing it to bounce off reflective surfaces. Hang a large mirror directly across from the largest window in your room, or arrange an array of smaller mirrors to help brighten a dark staircase.

29 Infinity Mirrors Artwork Yayoi Kusama Yayoi Kusama (Japanese artist) had a breakthrough in 1965 when she produced Infinity Mirror Room Phalli s Field. Using mirrors, she transformed the intense repetition of her earlier paintings and works on paper into a perceptual experience.

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32 Want to see more? Check out the website: -exhibition/

33 Standing between two mirrors (French: Mise en abyme) Most mirrors reflect ~90% of the light that they receive, so as the images continue on, they slowly become darker.

34 Example from page 447 in your textbook of an object placed between two flat mirrors. Again, notice how each virtual image becomes a bit darker than the last.

35 Image location can be predicted with ray diagrams The relationship between the object distance from the mirror, which is represented as p, and the image distance, which is represented as q, is such that the object and image distances are equal. Similarly, the image of the object is the same size as the object.

36 Practice How to Draw a Ray Diagram (you will have to copy this down in your notes) Let's imagine that a girl is looking at a mirror with her left eye closed and there is a green arrow in front of the mirror as shown to the left. How is the light reflected into her eye? Written instructions can be found here /Lesson-2/Ray-Diagrams-for-Plane-Mirrors