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PHYS 242 Lab

Reflection and Refraction In this lab we will investigate reflection and refraction in various materials, using a simulation from the PhET team. The simulation is available at the following link: https://phet.colorado.edu/sims/html/bending-‐light/latest/bending-‐ light_en.html

The simulation can be run in a browser. If you have issues with the simulation, try using another browser. If you are unable to run the simulation, your TA will provide you with remote assistance. When you run the simulation, choose the “More Tools” option.

NOTE: For this activity, you do not need to write a full lab report. Instead, answer the questions that are included in this lab. Please type up your answers, if possible. For the sketches, you can take pictures and include the images in your write up. Submit your answers to these questions on as a PDF. If you need help with this process, please ask your TA for assistance.

Part I: Basic Reflection and Refraction

When you begin the lab, be sure to select the “More Tools” option. Note that there are other options, for “Intro” and “Prisms.” If you select the wrong option, you can return to the “More Tools” version at the bottom of the screen.

This lab provides a laser pointer that shines from one medium into another medium. Check the “Normal” box at the lower left to show the normal line that is perpendicular to the surfaces. Turn the laser pointer on by clicking the red button. You will then see the beam travel through Medium 1 and hit the interface with Medium 2. One part of the beam is reflected back into Medium 1,

while the other is refracted in Medium 2. We can change the materials and the indexes of refraction on the right. The default version is air (n1 = 1.000) to glass (n2 = 1.500). You can also change the wavelength of the laser pointer in the upper left corner. In this box there are also options to see the light beam as a ray or a wave. For most of these problems, you will probably find it easier to use the “Ray” view. Finally, on the right side there are several tools you may find useful, including a protractor and probes to measure intensity, speed, and the oscillation of the waves.

For the first part of this simulation, use the protractor to set the incident angle of the laser beam at 60°. Set the wavelength of the laser pointer to 700 nm. Set Medium 1 (where the laser pointer is) to be air and Medium 2 to be glass.

Question: What is the angle of the reflected part of the beam? What is the angle of the refracted beam? Compare these to the expected values from the equations.

PHYS 242 Lab Week 13 Lab

Question: Use the intensity probe to measure the intensity of the incident, reflected, and refracted beams. Which is most intense? Does this make sense?

Question: Use the speed probe to measure the propagation speed of the beam in the two media. Does the speed change? Explain. Does this contradict the statement that “the speed of light is constant”?

Question: Select the wave option in the top left corner. This will show the propagation of the light waves, slowed down so that we can see the individual peaks and troughs. The time probe will

allow you to see the intensity at two points as a function of time. Based on this observation, what do you notice about the frequency and wavelength of the beam as it is reflected and reflected? Discuss.

Part II: Dispersion

Now change the wavelength of the laser pointer to 380 nm. Keep your incident angle set to 60° with an air to glass interface. Make sure you select the “Angles” box in the bottom left-‐hand corner of the screen

PHYS 242

Question: Do the angles of the reflected and refracted beams change from the values in Part I? Why?

Question: A laser pointer produces light at a single wavelength. Suppose instead that we were observing a source that produces light at all wavelengths, like the Sun. What would an observer inside the glass see if they looked up at the sunlight traveling through air?

Part III: Total Internal Reflection

Now switch the media, so that the laser pointer is in glass at the top, with air at the bottom. Keep the incident angle set to 60° and the frequency set to 380 nm.

Question: What are the angles of the reflected and refracted beams? Explain your observation, using Snell’s Law.

Question: What critical angle do you need to ensure that some light will be refracted from the glass into the air? Compare the value in the simulation with your calculated value.

PHYS 242 Lab Week 13

Part IV: Mystery Media

We will now determine the indexes of refraction for two Mystery materials, A and B. These materials can be selected from the list of materials on the last page of this lab. Be sure to set your laser pointer to a frequency of 589 nm.

Question: Devise an experiment for determining the indices of refraction for these. Explain your methodology. What are the indices of refraction for the two media?

Question: Below is a table of indexes of refraction from Serway & Jewett. Based on your predicted indices of refraction, can you identify any possible substances for Mystery A and Mystery B?

PHYS 242 Lab