PHY 172 Bending Light Lab

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Physics

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Dec 6, 2023

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Justin Alvarado Bending Light Remote Lab This lesson is designed for a student working remotely.) This lab uses the Bending Light simulation from PhET Interactive Simulations at University of Colorado Boulder, under the CC-BY 4.0 license. https://phet.colorado.edu/sims/html/bending - light/latest/bending - light_en.html Learning Goals A. Describe what happens to light when it shines on a medium. B. Explain light direction changes at the interface between two media and what determines the angle. C. Describe the effect of varying wavelength on the angle of refraction. D. Explain why a prism creates a rainbow. E. Apply Snell’s law to a laser beam incident on the interface between media. Everyday Physics in your life: I put a chopstick in a glass of water like in the picture on the right. What do you notice? Try this at home with some other things like a spoon or fork. Do you observe any different things? ( If you don’t have a clear glass, you can still see some interesting things. You might want to use a larger container like a bathroom sink). What ideas do you have about why things look different under water? Develop your understanding: 1. Explore the Intro screen to find some things that happen when light rays shine into water. Figure out how to test what happens when the light rays come from underwater. Compare your ideas about why things look different under water to how light rays appear to “bend.”. When light rays come from underwater, the light is changing its means of propagation because water is denser than air. When the light rays shine into water, the two mediums separate from their rectilinear propagation at the surface of separation and when this happens, light bends which is also known as refraction. This light bending occurs at specific angles, if we see the light with our eyes not bending, then the object is at its actual size. But as we go from 0 degrees to larger angles, the size of the object begins to change due to the bending of light rays. 4/15/20 Loeblein https://phet.colorado.edu/en/contributions/view/546 6 page1

Explain your understanding: 2. Experiment in the Intro screen using the tools to observe what happens to light when it shines on water. Use both the Ray and Wave models for light. a. Write about how each tool helps investigate the behavior of light. Both the ray and wave feature effect how light reacts with water and the index of refraction effects the angle at which the light shines through. b. Describe the behavior of light when it shines on water. Provide examples for a variety of incident angles to support your description. As light shines into water, the light is refracted because it is passing from a lower density area to a higher density area. At 30 degrees in air, light is refracted in the water at about 26 degrees. At 60 degrees, light is refracted in the water at 40 degrees. At 75 degrees, water is refracted in the water at about 47 degrees. c. What changes if the light is underwater and the light goes into air? If the light is underwater and goes into air, instead of the light slowing down and refracting toward the normal line, as it reaches the new medium the light will speed up and refract toward the surface of water. d. What changes if you shine light on glass? When shining light on glass, the index of refraction increased from 1.33 to 1.50, causing more of a bend away from the surface of the glass. The refraction in glass is higher than the refraction in water. 4/15/20 Loeblein https://phet.colorado.edu/en/contributions/view/546 6 page2

e. Test materials with other indexes of refraction to see if your ideas about the behavior of light seem comprehensive. The behavior of light seems consistent with this experiment. As the index of refraction increases, the higher the refraction is away from the surface medium. As the index of refraction decreases, the less amount of refraction there is and the light bends toward the surface medium. f. Write a summary description of what happens to light when it goes from one medium to another. Use the following vocabulary words in your summary: index of refraction, incident angle, reflected angle, and refracted angle. When light passes from one medium to another, refraction is taking place. Refraction is the bending of light and the center of the bending is known as the refracted angle. The index of refraction is the measure of the amount of refraction taken place. The angle of incidence is equivalent to the angle of reflection. Both the incident angle and reflected angle are the 2 angles from the medium where the ligh t is coming from, and the refracted angle is from the medium where light is passing through. 3. Use More Tools screen to observe variation in refraction as the wavelength (color) of incident light varies. a. The difference in refraction angles is small and difficult to detect in this simulation, so I have shown a possible experimental setup for you to try. How much does the angle of refraction change from 380nm to 700nm when the incident angle is 80? Run some of your own experiments to see if you can detect a difference in refracted angles at other incident angles. Why did I choose 80 degrees? At 80 degrees where the light is 380nm, the angle of refraction is 40.4 degrees. At 80 degrees at 700 nm, the angle of refraction is 41.1 degrees. This is a difference of .7 degrees. Now at 380nm at 16 degrees, the angle of refraction is 10.5 degrees. At 700 nm at 16 degrees, the angle of refraction is 10.6 degrees. This is a difference of .1 degrees. This shows that the difference is proportional to the angle of incidence. So as the angle increases the more of a difference of refraction between wavelengths. b. Use the Speed tool as you vary the wavelength to help understand why the angle of refraction varies. Describe the results of your experiments 4/15/20 Loeblein https://phet.colorado.edu/en/contributions/view/546 6 page3

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From the speed tool, the speed of light is slower in water due to the density of the medium. As the wavelengths increased underwater, the speed increase by about .1. Meaning longer wavelengths move slightly faster than shorter wavelengths underwater. This then means that longer wavelengths are refracted less than shorter wavelengths because shorter wavelengths face more obstruction in the medium they are traveling in. c. Does the color of the color of the light change during refraction or reflection? The color of light does not change during refraction or reflection because the frequency is the same, only wavelength changes. 4. Explore the Prisms screen to see how your understanding of refraction applies when light travels through a medium like glass. Give specific examples and images from the simulation to explain how your understanding applies. Simulation tip: Remember that the Normal dashed line and protractor will help take measurements.. From the prisms simulation, we can see that light refracts at the interface of a medium, light refracts toward the interface as the density of the environment increases, and that longer wavelengths refract less than shorter wavelengths. Refraction at interface Refraction with shorter wavelength 4/15/20 Loeblein https://phet.colorado.edu/en/contributions/view/546 6 page4

Refraction with longer wavelength Refraction with denser enviroment 5. Experiment to find ways to make rainbows. a. Insert at least one setup where light passing through a prism gives a rainbow and describe why a rainbow is formed. As light passes through a prism, it slows and refracts, but different wavelengths bend at different angles. This then separates the light into different wavelengths which causes a rainbow of colors. b. Explain why only some types of light will yield rainbows. The light cannot be monochromatic because the light needs the colors of the rainbow to produce said rainbow. The prism separates the light into different colors because the wavelengths bend at different angles. Test your understanding and self-check Open the full Bending Light simulation 6. Show that you can use Snell’s Law ( n 1 sinΘ 1 = n 2 sinΘ 2 ) to predict the angle of reflection and angle of refraction for several scenarios. Show your work. After you have completed the calculations, use simulation to check your work For incident angle of 30 degrees light shining a. from air into water 1.00(sin30) = 1.33(sin Θ 2 ) Θ 2 = 22.082 degrees b. from water into air 1.33(sin30) = 1.00(sin Θ 2 ) Θ 2 = 41.68 degrees c. from air into glass 1.00(sin30) = 1.50(sin Θ 2 ) 4/15/20 Loeblein https://phet.colorado.edu/en/contributions/view/546 6 page5

Θ 2 = 19.47 degrees d. from water into glass 1.33(sin30) = 1.50(sin Θ 2 ) Θ 2 = 26.316 degrees e. from air into a medium with an index of 1.22 1.00(sin30) = 1.22(sin Θ 2 ) Θ 2 = 24.19 degrees. 4/15/20 Loeblein https://phet.colorado.edu/en/contributions/view/546 6 page6

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