# Does the law of reflection hold true for all the angles in the table and plot or were there points which deviated significantly from a straight line? Were there any angles at which light was not reflected? Explain.

ON-LINE LAB #4 NAME:

Reflection and Refraction NAU User ID:

Introduction

The law of reflection describes the motion of waves and objects that strike smooth surfaces. It states that the angle of incidence is equal to the angle of reflection. This law can be applied not just to beams of light, but also to billiard balls rebounding off the bumper on a pool table, sound waves echoing off a wall, and even water waves reflecting off the side of a boat. The law of refraction describes the motion of a wave as it travels from one medium to another, such as air to water, or water to glass, etc. While slightly more complicated, it allows us to calculate the direction of waves as they travel from one medium to another.

Instructions

This lab is composed of three tasks which utilizes a PhET (Physics Education technology) simulation.

Go to the web page https://phet.colorado.edu/en/simulation/bending-light.

Click on the Bending Light icon. This will take you to the following page. The More Tools simulation is the only one that we will be using in this lab (though if you have the time you may enjoy exploring the other simulations too). Click on the More Tools icon. It will take you to the following page. Take some time to familiarize yourself with the simulation.

· The upper and lower materials can be changed via the sliders and/or the drop-down menus in the boxes on the righthand side.

· In the upper left box, the selector buttons allow you to switch between the Ray and Wave models while the slider allows you to change the wavelength of the laser.

· The bottom left box contains tools that can be placed on the lab bench for use.

· When the Normal box is checked in the lower left corner, the normal line is sketched, and when the Angles box is checked, the angles of incidence, reflection, and refraction are indicated in the figure.

· The laser can be turned on and off via the red button on the laser. Once you are familiar with how everything works, click the reset icon in the lower right corner and proceed to Task #1.

PHY 112, 172, 262 ON-LINE LAB

Check the Angles box and switch the laser on. Note the incident ray, reflected ray, and refracted ray. In this task, we will investigate the relationship between the incident and reflected rays.

2

Drag the laser such that the incoming ray makes and angle of 5 degrees with the normal line. Read the angle of reflection and record it in the table below. Repeat the procedure for all the incident angles listed in the table.

Once you have recorded all the reflected angles, you are ready to plot the data in a graph.

(See the appendix at the end of this document for a simple way to create and insert a graph.)

Once you are satisfied with your graph you can copy and paste it in the box to the right of the table.

Insert a copy of your graph in the box below

 Angle of Incidence (degrees) Angle of reflection (degrees) 5.0 10.0 15.0 20.0 25.0 30.0 35.0 40.0 45.0 50.0 55.0 60.0 65.0 70.0 75.0 80.0 85.0

Questions

1. Note the shape of your graph. It should look linear. We now have experimental evidence for the law of reflection (Angle of Incidence = Angle of Reflection).

Does the law of reflection hold true for all the angles in the table and plot or were there points which deviated significantly from a straight line? Were there any angles at which light was not reflected? Explain.

2. Use the drop-down menu to change the bottom material to water. Repeat the procedure using the incident angles listed in the table below and record the angle of reflection in the table. Does the law of reflection still hold true? Explain.

 Incident angle (degrees) Reflected angle (degrees) 15 30 45 60 75

3. Using the drop-down menus, change the bottom material to air and the top material to glass. Repeat the procedure using the incident angles listed in the table below and record the angle of reflection in the table. Does the law of reflection still hold true? Explain.

 Incident angle (degrees) Reflected angle (degrees) 15 30 45 60 75

4. Using the drop-down menus, change the top material to air and the bottom material to glass. Using the wavelength slider, change the wavelength to a value different from the default value of 650 nm. Record the wavelength and color of the laser. Wavelength = nm Color =

Repeat the previous procedure, using the incident angles listed in the table below, and record the angle of reflection in the table. Does the law of reflection still hold true? Explain.

 Incident angle (degrees) Reflected angle (degrees) 15 30 45 60 75

5. Write a short paragraph summarizing the results of this task. Does the law of reflection seem to depend on the materials or wavelength of light? Does it seem to hold over all angles?

PHY111, 161 ON-LINE LAB

Reset the simulation, check the Angles box, and switch the laser on. Note the incident ray, reflected ray, and refracted ray. In this task, we will investigate the relationship between the incident and refracted rays.

2

Drag the laser such that the incoming ray makes and angle of 5 degrees with the normal line. Read the angle of refraction and record it in the table on the following page. Repeat the procedure for all the incident angles listed in the table. you are ready to plot the data in a graph.

(As you did in part one above, see the appendix at the end of this document for a simple way to create and insert a graph.)

Once you are satisfied with your graph you can copy and paste it in the box to the right of the table.

Insert a copy of your graph in the box below

 Angle of Incidence (degrees) Angle of Refraction (degrees) 5 10 15 20 25 30 35 40 45 50 55 60 65 70 75 80 85

Note the shape of your graph. If you look carefully, it should NOT appear linear as our reflection graph did, but curved, opening downward and flatter on the top. Thus, we have shown that refraction does NOT follow the simple linear relationship that reflection does.

Questions

1. The refractive property of a material can be characterized by its index of refraction.

Set the laser to an incident angle of 30 degrees and use the slider to set the index of refraction of the bottom material to 1.600. Use the Speed Tool

to measure the speed of the refracted light beams by placing the probe tip in the path of the beam. In the table below, record your value as some decimal number value of c

(Note: the speed of light changes depending on the material it is passing through. c represents the speed of light in a vacuum.)

Repeat the procedure for all the indices of refraction in the table. Note that you will have to move the probe tip as the refracted beam moves.

How does the speed of light in the material depend on the index of refraction of the material?

 Index of refraction of bottom material Speed of light in bottom material 1.600 1.500 1.400 1.300 1.200 1.100 1.000

2. Drag the Speed tool back to its storage box. Use the slider to change the index of refraction of the bottom material. How does the angle of refraction depend on the index of refraction of the bottom material? Explain.

3. Reset the bottom material to glass and use the slider to change the index of refraction of the top material. How does the angle of refraction depend on the index of refraction of the top material? Explain.

4. Reset the top material to air and the bottom material to glass and use the wavelength slider to change the wavelength of the laser. Does the angle of refraction depend on the wavelength of light? Explain.

5. The law of refraction, also known as Snell’s law , is given by . Here, is the index of refraction of the top material, is the index of refraction of the bottom material, is the incident angle, and is the refracted angle. We will not ask you to derive the equation at this point. Instead please just watch this brief YouTube video to help you understand it:

https://youtu.be/8wYkgZKboss

Now suppose a light ray is striking a glass block whose index of refraction is 1.600 at an incident angle of 72.0 degrees. We can use the law of refraction to solve for the refracted angle.

Here , , and . To solve for , we take . Next, we take the arcsine of this number. In the end, we arrive at 36.4706 degrees.

Now using the simulation, set the laser wavelength to 650 nm and the incident angle to 72.0 degrees. Record the angle of refraction here: Angle of Refraction = degrees.

Do the two angles agree?

6. Write a short paragraph summarizing the results of this task. Does the angle of refraction seem to depend on the materials or wavelength of light?

Questions

1. Back to the simulation. Set the index of refraction of the top material to 1.600 and the bottom material to 1.000. Adjust the laser to a variety of incident angles. Are there any incident angles for which there are no refracted rays? Explain.

2. Carefully determine the smallest angle for which we only have a reflected ray. Record that angle here. Critical angle = degrees

3. Here we know that , and . What should we use for ?

When the incident ray is at the critical angle, the refracted ray is actually traveling at 90 degrees (i.e. along the surface). Therefore, we use . Using Snell’s law gives:

4. The video found at https://www.youtube.com/watch?v=Lic3gCS_bKo provides a good qualitative summary of total internal reflection. Watch it and write a short paragraph summarizing the results of Task #3.

Save this document and return it to your TA via BB Learn

Appendix

Making a Line Graph and Inserting it in your MS Word Document

There are any number of ways that you can create, copy, and insert a simple line graph into an MS Word document like this one. If you already have a method that you are comfortable with and prefer, please feel free to use it. For instance, you may already have a graphing program on your computer, or you might like graphing directly from an Excel document. There are also many free online graph makers and if you have one you already like, and you can use that if you wish. In fact, you could even graph the data by hand on graph paper, take a picture of it with your phone and then paste the picture in the designated spots, and that would be perfectly fine too.

HOWEVER, here is one simple and easy method that you can use:

(These are step-by-step instructions that many of you may already will know how to do.)

Go to the following website:

http://www.shodor.org/interactivate/activities/SimplePlot/

The screen should look like this: The nice thing about this program is that you can copy and paste all your data into the graph generator at one time.

Once you have your data recorded in the table, simply highlight it all and copy it to the clipboard: Now, return to the online graph generator and paste the data into the data table You can also include grid lines and give the graph a proper title.

Next, right-click on the graph and click on View Image: A “copy-able” image will now appear. Right-click on the image and Copy Image. The final step is to paste the image into the appropriate location in your lab document: