# Physics

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Physics 104 Lab 9 REPLACEMENT: More about light Authors: B. Mullan, T. Loblein, N. Quarderer Objective: Investigate principles of optics, including how lenses use refraction to bend rays of light to form images, and how the wave properties of light produce interference and diffraction patterns INTRODUCTION Now that we understand a little about refraction, we can use this knowledge to examine a key application: refraction of light rays by lenses, specially shaped objects designed to bend light in key ways. This week we will begin to experiment with how lenses to produce images and develop an understanding of what affects them. In the next part of the lab, we will explore a different property of light. So far, we’ve been treating light as rays, but that is an incomplete picture. Light is really a wave (and something else, too, but we’ll worry about that later!). Light waves propagate in the direction of rays, but they are made up of oscillating electric and magnetic fields. We are going to experiment with these waves, first by comparing them to water and sound waves (waves we may be more familiar with), and then seeing what happens when waves encounter each other and add together. We call this interference. Light can also diffract – essentially, scatter – off a boundary, like an aperture or a single slit (or any other opening, for that matter). Note: This is a temporary, online-only replacement lab for Lab 9. It can be completed independently, or in small lab groups (of 2-3 students, maximum). If students choose to work in groups, they must coordinate how to do so remotely. However, each student must complete their own labwork individually and must submit their work according to their instructor’s directions. PART 1: LENSES AND GEOMETRIC OPTICS Open the following simulation:

https://phet.colorado.edu/en/simulation/geometric-optics and click on the “play” symbol to open (or download the file). You will need to use a browser that can handle flash or download the simulation. Note that for this part of lab, the “ray model” of light will be used. This means that we will treat light as a simple ray instead of a propagating wave of electric and magnetic fields. Take the pencil and raise it so that the eraser is sitting on the principal axis. Click on the “principal rays” button.

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Questions to answer: Use the simulation to answer the questions below (no error analysis).

1) Draw the ray diagram you see. What does the “X” symbol signify?

2) Describe in detail what you notice about the three special principal rays: how do they enter the lens, and how do they exit the lens?

3) There are several properties of the lens you can change in this simulation. For each

one below, vary its value and describe what its effect on the image (its size, location, and brightness), and why you think that effect occurs:

a) Radius of curvature of the lens

b) Refractive index

c) Lens diameter

4) You can change the location of your object (the pencil). Drag the pencil so that it is

farther away from the lens. Explain the result.

5) Drag the pencil so the eraser is right on top of the focus. Draw the ray diagram.

a) What happens to the principal rays that enter the lens?

b) Will they ever form an image?

6) Drag the pencil so it is inside the focus. Draw the ray diagram.

a) Will the rays ever form an image, and if so, where? Click on “virtual image” to check your answer.

b) What do you suppose “virtual image” means, then?

c) Imagine that you are looking through the lens from the right-hand side. What

PART 2: WAVES AND INTERFERENCE Open the following simulation:

https://phet.colorado.edu/en/simulation/wave-interference and click on the “play” symbol to open (or download the file). You will need to use a browser that can handle java if you use the original version. Before you begin, you will need to experiment with the simulation to see what it does and how you can control it. Don’t be afraid to click around and try things out!

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Questions to answer: Use the simulation to answer the questions below (no error analysis). 2-1: Water Waves: start with the “Water” tab of the simulation

7) How does changing the frequency of the drip (disturbance) affect the characteristics of the water waves?

8) How does changing the amplitude of the drip affect the characteristics of the water

waves?

9) Sketch the water waves from both the top and side views. Label the wavelength of the wave in each of your sketches.

10) How does the wavelength of the wave depend upon the frequency of the wave? How

were you able to come to this conclusion?

11) Come up with a method to determine the speed of a water wave. Outline your procedure and share your results. You may want to consider running several trials and averaging your results.

12) How does changing the frequency of the drip affect the speed of the waves?

13) What effect does changing the amplitude of the drip have on the speed of the waves?

14) How does amplitude change with distance from the disturbance? How could you

tell? What might be causing this to happen?

15) What do you think would happen to the waves if a barrier with a narrow slit (aperture) were placed in the path of the wave? Test it out and comment.

16) As waves pass through an appropriately-sized aperture they can spread out, or

diffract; something you just observed. Predict what would happen if you increased the size of the aperture. Test it out and describe what happens.

2-2: Sound Waves: now go to the “Sound” tab of the simulation

17) Using the method you developed in the previous activity, measure the speed of a sound wave. Compare this with the ‘accepted’ value of the speed of sound (you may need to go online or elsewhere to find this value). What might be some reasons for any discrepancies between your calculated value and the accepted value?

18) How does the sound intensity (amplitude or ‘volume’) change as the distance

between the source of the sound wave and the observer increases? How could you tell? What might be causing this to happen?

2-3: Light Waves: now go to the “Light” tab of the simulation

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19) Use the simulation to determine the wavelengths of the following colors of light that are used in the simulation (hint: what is the definition of a wavelength?):

Red: Orange: Yellow: Green: Blue: Violet:

20) Using the method you developed in the previous activities, determine the speed of a

light wave. Does this speed depend on the frequency of light? Amplitude?

21) Compare this with the ‘accepted’ value of the speed of light (you may need to go online or elsewhere to find this value). What might be some reasons for any discrepancies between your calculated value and the accepted value?

22) You saw in the previous activity that waves can diffract when they interact with an

appropriately-sized aperture. Light, being a wave, can do the same thing. Sketch what you predict the pattern would look like on a screen placed in the path of a light wave if:

a. Light hits screen unobstructed without passing through aperture: b. Light hits screen after passing through a single narrow aperture (single slit): c. Light hits screen after passing through two, closely-spaced, narrow apertures

(double slit):

24) You just saw that the pattern of light after passing through a double-slit resembles a

series of closely-separated bands of light, known as an interference pattern. Describe what happens when the following parameters are adjusted independently:

a. Wavelength of light (λ):

b. Distance between barrier and light source (L):

c. Distance between slits (d): SUMMARY

Concisely and accurately summarize your main findings in this lab. What have you learned about ray optics, interference, and diffraction of light?

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