PhysicsThe Nature of Light

Experiment: The Polarization of Light

PurposeThe purpose of this experiment is to demonstrate ways in which light can be polarized and to illustrate several uses of polarized light.

Equipment

2 Polaroid filterscrystal of calcite (Iceland spar)text or magazine with shiny covercellophane strips mounted on a glass

plate as shown in Figure 1*calculatorclear plastic protractor (or ruler)

Figure 1*Note Inexpensive cellophane tape or Scotch brand packing tape usually works. To check if a particular tape will work, tape two or three layers on a glass slide. Rotate the slide between two Polaroid lenses. If colors appear, and change when the lenses are rotated, the tape is the kind needed.

Procedure

Part One: Production and Detection of Polarized Light with Polaroid Filters1. Look through one of the Polaroid filters at a light source. Rotate the filter.

2. Do the same with the other filter.

3. Now hold both filters together, and rotate one of them while you look through them at the light source.

Part Two: Production of Polarized Light by Crystals4. Look through a transparent piece of calcite (Iceland spar) at the period at the end

of this sentence.

5. Slowly turn the crystal.

6. Now look through a single rotated Polaroid filter at the images in the crystal.

Side View

Top View Top View

Experiment: The Polarization of Light, Procedure, page 2

Part Three: Production of Polarized Light by Reflection7. Hold a textbook or a magazine with a shiny cover so that the light shinning on it

makes a glare. Look at the surface through one Polaroid filter as you rotate the filter.

8. Place a vertically polarized filter over one eye and a horizontally polarized filter over your other eye. Look at the glare. Close first one eye and then the other.

Part Four: Use of Polarized Light to Observe Interference Phenomena9. Cross the two Polarized filters and place between them the glass slide containing

the cellophane stripes of verifying thickness. Rotate the glass slide until you observe the brightest colors.

10. Rotate the Polaroid filters so that their planes are parallel. Again rotate the glass slide until you observe the brightest colors.

Part Five: Use of Polarized Light to Determine Structural Strains11. Use a polarizing filter to observe a liquid crystal display on a calculator. Rotate the

filter.

12. Place a clear, plastic protractor (or ruler) between two polarizing filters. Look at an incandescent light source with this. Do a complete rotation of one of the filters. Position the two filters the same way that produced no light in Procedure 3 (Part One).

Name _______________________ Class __________ Date ______________

PhysicsThe Nature of Light

Experiment: The Polarization of Light

Observations

Part One: Production and Detection of Polarized Light with Polaroid Filters1. As you rotate the first Polaroid filter, does the intensity of light vary?

2. Do you get the same result with the second Polaroid filter?

3. How does the intensity of light coming through a single filter compare with that reaching your eye directly?

4. What effects do you observe when you are looking through both Polaroid filters?

5. How much do you have to turn one filter to go from maximum to minimum brightness?

Part Two: Production of Polarized light by Crystals6. What do you see when you look through the calcite?

7. What movement do you observe when the calcite is rotated?

8. What happens as the single Polaroid filter is rotated?

Part Three: Production of Polarized Light by Reflection9. What happens to the glare as the Polaroid filter is rotated?

10. Which filter reduced the glare best? (Answer this using the polarizing plane, not the eye.)

Experiment: The Polarization of Light, Student Handout, page 2

Part Four: Use of Polarized Light to Observe Interference Phenomena11. Compare the colors when the Polaroid filters are crossed to when they are parallel.

12. What colors are associated with each thickness?

Part Five: Use of Polarized Light to Determine Structural Strains13. What did you observe when you looked at the liquid crystal display as one filter is

rotated?

14. Is the light from liquid crystal displays polarized? How do you know?

15. What did you observe when one of the filters was rotated with the protractor between the two filters?

Concluding Questions16. When the Polaroid filters are placed so that a minimum of light passed through, the

filters are said to be crossed. Explain why crossed polarizes transmit a minimum of light.

17. Explain your observations in Part Two.

18. In Part Three, what plane is the glare reflected from a polished surface polarized? How can you tell?

19. What cause the different colors in Part Four?

20. In Part Five, why can the clear plastic protractor be seen between two crossed Polaroid filters even through nothing else can be seen through the filters?