Lab 9: Polarization of Light

The purpose of this lab is to explore how light rays can be polarized in different situations. There will be Reflection Questions throughout this Lab instead of concentrated at the end.

Part 1: Dependence of Radiation Intensity on Distance For this first part, we'll use the Solar System Energy Lab from the Physics Aviary (http://www.thephysicsaviary.com/Physics/Programs/Labs/IntensityLab/). We'll learn about how much radiation (light energy) the rocket can receive at different distances from the Sun.

1. Before you begin, please read the information on the main page. When you're ready to proceed, click Bick,

2. Our rocket starts our near Mars's orbit. Use the curling arrows to point the tip of the rocket at the Sun. Once you've got this lined up, click Pause and then click Data. What value do you read for the Intensity in W/m²?

3. Use the forward arrow () to increase the rocket's speed and the backward arrow () to slow the rocket down. Move the rocket around on the screen and take data at each of the locations listed. Ignore the middle column of Area for now. We'll say the initial intensity (I₀) occurs where we start near Mars's orbit.

Location	Area (A, m2)	Intensity (I, W/m2)
Mars's Orbit	2 m2	I0=
Midway between Mars's and Earth's Orbits	2 m2
Earth's Orbit	2 m2
Midway between Earth's and Venus's Orbits	2 m2
Venus's Orbit	2 m2
Midway between Venus's and Mercury's Orbits	2 m2
Mercury's Orbit	2 m2
Midway between Mercury's Orbit and the Sun	2 m2
Near the Sun	2 m2

4. We define intensity as the power per unit area . How does the power of light (P, measured in W) compare at three different locations from the table above if we say the area of the rocket's sensor is 2m²?

L₁:

L₂:

L₃:

5. How would the power of light change at those locations if the area was 3m²? What if it was 0.5m²?

6. How does the intensity of a source depend on distance? From your data, what kind of mathematical relationship would you say intensity has with distance? Explain why you used this relationship.

Part 2: Polarization of EM radiation An electromagnetic wave consists of Electric and Magnetic fields that oscillate at right angles to each other, and at right angles to the direction of wave motion. The electric field is particularly important" whenever the wave strikes a piece of matter the electric field produces a force on the charged particles in the matter given by . Because E is oscillating in time, the charged particles are forced to oscillate. The direction of the electric field is called the polarization axis of the light wave. The intensity of the radiation is proportional to the square of the average Electric field.

where c is the speed of light, very close to 3 x 10⁸ m/s. A polarizing filter is made of special plastic material. Light emerging from the filter is polarized parallel to the filter's transmission axis. Light that strikes the filter will be completely transmitted if the incoming light is polarized along that axis. The light will be absorbed if in the incoming light is perpendicular to that axis. For angles in between those values, there is partial absorption and partial transmission.

1. Describe what has to happen for light to be completely transmitted by a polarizing filter.

2. Describe what has to happen for light to be completely absorbed by a polarizing filter.

3. What has to be true about light transmitted through the filter?

4. Explore the polarizer graph at the following site (http://physics.bu.edu/~duffy/HTML5/polarized_light_onepolarizer.html). What do you see on the graph with the initial settings? Rotate the incident lightwhat changes? Now return the incident light to 0 degrees and rotate the polarizer. What's changing this time?

5. Return both incident light and polarizer to 0 degrees. Now move them move, changing the angles in varying amounts. What do you notice happening? What does this tell you about the polarization of the light?

6. Repeat steps 4 and 5 now using three polarizers (http://physics.bu.edu/~duffy/HTML5/polarized_light.html). What's different about the polarization of light in these scenarios compared to one polarizer? What might this tell you about the polarization of light?

7. Now we're going to go to a new Polarization Lab (http://labman.phys.utk.edu/3D%20Labs/polarization/polarizationlab.html). You are presented with a He-Ne laser, an optical breadboard, and several optical components.The components can be dragged to different positions on the breadboard and the components can snap to the holes on the breadboard which have a spacing of 1 unit = 2.5 cm.The screen is 4 units wide and 2 units high and the lines on the screen are spaced by unit.Clicking anywhere on the breadboard you can rotate and zoom the view. A transparent viewer can help you align the beam.All components can be translated and rotated, and the post holders can be raised and lowered with the mouse.

8. The He-Ne laser beam ( = 633 nm) is unpolarized.You have 3 (ideal) linear polarizers and a power meter with a Bluetooth connection to a digital readout to monitor the beam intensity.You can also monitor the intensity of the laser beam on a screen (corrected for the average response of the human eye to light).The polarizers are mounted in a rotation stage.The transmission axis of the polarizers is horizontal when the rotation stage reads zero.You can explore what happens to the transmitted intensity if you rotate a third polarizer between two crossed polarizers and you can verify the Law of Malus.You can also reflect the laser beam off a glass slide and explore polarization by reflection.The back of the glass slide is painted black to absorb the transmitted portion of the beam.The slide is made of glass with ahigh index of refraction (you can measure the index of refraction by finding theBrewster angle).

9. If light propagates through a transparent material such as water or glass, it interacts in various ways with the atoms or molecules that make up the material.This interaction can be wavelength and polarization dependent.Due to the interaction, light moves through a transparent material with an apparent speed .The index of refraction n is a property of the material.It is greater than 1, so that v is less than c.In most transparent materials the index of refraction depends slightly on the wavelength of the light, and in some materials, it depends on the polarization.

10. Linear polarization:An ideal linear polarizer is a material that passes only light waves for which the electric field vector is parallel to its transmission axis.IfE₀is the incident field vector and the angle betweenE₀and the transmission axis is , then the magnitude of transmitted field vector is , and its direction is the direction of the transmission axis.The intensity I of an electromagnetic wave is proportional to the square of the magnitude of the electric field vector.We therefore have the Law of Malus, which is

11. Spend a few minutes exploring what happens to the intensity of the beam spot on the screen as you pass the beam first through one and then through two polarizers like in the figure below.Observe the change in transmitted light intensity as either of the polarizers is rotated.This combination acts as a "light valve." Describe your observations here.

12. Place a third polarizer between two crossed polarizers and rotate the transmission axis of the third polarizer while observing the intensity of the beam spot on the screen as in this figure. Describe your observations below.

13. Now that you've had the chance to explore the simulation, experiment with it. In this experiment you will use a linear polarizer to produces apolarized beam and then pass this beam through a second polarizer whose transmission axis makes an angle with respect to the transmission axis of the first one. We'll verify the Law of Malus, above.

14. Use the setup shown in the picture below.It uses the power meter to monitor the beam intensity.You can increase the spacing between the components to make it easier to rotate the polarizers.Start with the transmission axes of both polarizers horizontal (zero degrees).If necessary, adjust the height of all components so that the beam falls onto the power meter and the meter reads 0.9 mW.(Note:the readout position is fixed, you cannot drag it around.)

After the laser light has passed through the polarizers, it is almost entirely 100% polarized along the horizontal direction, since the transmission axis of each polarizer is horizontal. Record your data in the first row of the table.

Angle (degrees)	Intensity (mW)	(mW)
0	I0=
20
40
60
80
100
120
140
160
180

15. Now we're going to rotate the second polarizer with respect to the first one in 20 increments, stopping to measure the Intensity every 20, which go into the second column.

16. Then calculate the values in the third column, using the I₀ you measured when both polarizers were aligned.

17. Discuss your results.Have you verified the Law of Malus?How can you tell?No experiment is perfect, there are always uncertainties.Are your uncertainties small enough so that you can tell one way or the other? (Note: Making a quick graph of angle vs. may help you answer this one.)

Part 3: Polarization by Reflection Whenever light strikes a transparent material, some of the light is reflected and some is transmitted into the material (refracted). The incident intensity is split into reflected and refracted intensities. Regardless of polarization, light reflects from a surface according to the law of reflection: angle of incidence equals angle of reflection. Ordinary light is a mixture of polarizations. Some polarizations are efficiently reflected from a surface, so that reflected light tends to be polarized. There is an angle of incidence for which all of the reflected light is polarized along a single axis. This angle is called Brewster's Angle or sometimes the polarizing angle, and that angle for light reflecting off of glass is between 55 and 60.

1. We'll continue to use the Polarization Lab from above, but refresh the page to easily reset it back to the initial settings.

2. You will reflect the laser light off a glass slide.You will make sure that the incident angle is close to the Brewster angle and verify that light polarized in the plane of incidence it will not be reflected at the Brewster angle. The plane of incidence is a plane perpendicular to the reflecting surface that contains the incident beam.

3. If the reflecting surface is horizontal (for example, if sunlight is reflecting at a low angle off a lake), then the plane of incidence is vertical. The reflected light is horizontally polarized and can be blocked by a polarizer with a vertical transmission axis.If the reflecting surface is vertical, as for our glass slide, then the plane of incidence is horizontal, and horizontally polarized light will not be reflected at the Brewster angle.Then the reflected light is vertically polarized and can be blocked by a polarizer with a horizontal transmission axis.You will reflect the laser light off a vertical glass slide and find the Brewster angle.

4. Set up the components as shown below.Rotate the glass slide post by an angle between 55 and 60 degrees.Rotate the posts of the screen and a polarizer by the same amount in the opposite direction.

5. Position the screen so that you can see the reflected beam on the screen. You can use the viewer to help you find the reflected beam.

6. Remove the viewer and put the polarizer in its place.Make sure that the beam passes through the polarizer and you can still see it on the screen.If you need a finer position adjustment to accomplish this, uncheck snap to grid in the upper right corner of your screen.

7. Rotate the polarizer in front of the screen by 90^oto 180^o, so that its transmission axis is horizontal. It now blocks vertically polarized light.

8. If, with the transmission axis horizontal, you can still see a faint beam on the screen, try a slightly different angle for the glass slide post.When you adjust this angle make sure that the beam still passes through the polarizer onto the screen. Rotate the polarizer transmission axis back to vertical to check.You may have to adjust the position of the screen and the polarizer.

9. Iterate until the reflected intensity is practically zero and you can no longer see the beam on the screen with the polarizer transmission axis horizontal, but you can see the beam when the polarizer transmission axis is vertical.Take a screen shot and paste it here.

Screen shot:

10. Record the glass slide post angle for this situation. This is the Brewster angle _Bfor the glass slide in air.

_B=

11. Use this measured value to find the index of refraction of the glass slide. We have tan_B = n_glass.Record n_glass.Is your value for n_glassreasonable?

n_glass=

12. Keep the rotation stage fixed at the Brewster angle and rotate the polarizer.You should see a reflected beam.At the Brewster angle the reflected beam is 100% vertically polarized.

13. Based on what you've learned in this lab, how do you think glare reducing sunglasses work?

121 6 0 56 ..Ill 96%| 6 Lab9-Polariz CT} /0 Lab 9: Polarization of Light The purpose of this lab is to explore how light rays can be polarized in different situations. There will be Reflection Questions throughout this Lab instead of concentrated at the end. Part 1: Dependence of Radiation Intensity on Distance For this first part, we'll use the Solar System Energy Lab from the Physics Aviary (httg:[fwwwtheghysicsaviary.com1PhysicsfProgramsgLabsilntensityLab. We'll learn about how much radiation (light energy) the rocket can receive at different distances from the Sun. 1. Before you begin, please read the information on the main page. When you're ready to proceed, click Bick, 2. Our rocket starts our near Mars's orbit. Use the curling arrows to point the tip of the rocket at the Sun. Once you've got this lined up, click Pause and then click Data. What value do you read for the Intensity I'n W/mz? 3. Use the forward arrow (9) to increase the rocket's speed and the backward arrow (6) to slow the rocket down. Move the rocket around on the screen and take data at each of the locations listed. Ignore the middle column of Area for now. We'll say the initial intensity (Io) occurs where we start near Mars's orblt. Location Area (A, m2) Intensity (I, W/mz) Mars's Orbit 2 m2 In: Midway between Mars's 2 m2 and Earth's Orbits Earth's Orbit 2 m2 Midway between Earth's 2 m2 and Venus's Orbits Venus's Orbit 2 m2 Midway between 2 m2 Venus's and Mercury's Orbits Mercury's Orbit 2 m2 Midway between 2 m2 Mercury's Orbit and the Sun Near the Sun 2 m2 :a: 2 A Print Layout Headings Edit Share Read Aloud III C) l. We'll continue to use the Polarization Lab from above, but refresh the page to easily reset it back to the initial settings. 2. You will reflect the laser light off a glass slide. You will make sure that the incident angle is close to the Brewster angle and verify that light polarized in the plane of incidence it will not be reflected at the Brewster angle. The plane of incidence is a plane perpendicular to the reflecting surface that contains the incident beam. 3. If the reflecting surface is horizontal (for example, if sunlight is reflecting at a low angle off a lake), then the plane of incidence is vertical. The reflected light is horizontally polarized and can be blocked by a polarizer with a vertical transmission axis. If the reflecting surface is vertical, as for our glass slide, then the plane of incidence is horizontal, and horizontally polarized light will not be reflected at the Brewster angle. Then the reflected light is vertically polarized and can be blocked by a polarizer with a horizontal transmission axis. You will reflect the laser light off a vertical glass slide and find the Brewster angle. 4. Set up the components as shown below. Rotate the glass slide post by an angle between 55 and 60 degrees. Rotate the posts of the screen and a polarizer by the same amount in the opposite direction. 5. Position the screen so that you can see the reflected beam on the screen. You can use the viewer to help you find the reflected beam. 6. Remove the viewer and put the polarizer in its place. Make sure that the III C)