Basic Polarized Light

Introduction

Explore the wonders of polarization, birefringence and stress using a polarizer, an analyzer, an overhead projector and other common items.

Concepts

• Polarized light
• Transverse waves
• Birefringence
• Electromagnetic waves
• Refraction

Background

Light (electromagnetic waves) moves as transverse waves that oscillate in various directions, perpendicular to the direction of the wave motion. Polarizing filters, like those used in sunglasses, contain parallel slits that allow only light that is oscillating in the plane parallel to the slits to pass through. The light waves at angles different from the parallel slits are either reflected or absorbed. The intensity of the transmitted light is reduced.

If two polarizing filters are placed in the path of the light and the slits are aligned in parallel, then the light will pass through both filters. If the slits of the second filter are perpendicular to the first, no light will pass through the second filter. If the slits of each filter are at an angle to each other, then the component of the light wave parallel to the slits will travel through the filter with reduced intensity. As the angle between the slits of the two filters increases, the light intensity decreases. The second polarizer can determine the orientation of the polarization of light and is therefore referred to as an analyzer (see Figure 1). 

Polarization can also occur as a result of reflection and refraction. When light is reflected off of nonmetallic surfaces, such as pavement or a body of water, the reflected light can be partially polarized. If a beam of light passes from one medium to another, polarization can occur as a result of the beam changing direction. A form of the mineral calcite, called Iceland spar, will refract light into two perpendicular rays. This effect is known as birefringence (see Figure 2).

Sound waves differ from light waves in that sound travels in longitudinal or compression waves—the oscillations are parallel to the direction of the wave motion. Sound waves, unlike light waves, cannot be polarized.

Materials

Safety Precautions

Although the materials included in this kit are considered nonhazardous, please follow all normal laboratory safety procedures.

Disposal

The materials in this kit may be stored for future use.

Procedure

1. Obtain two polarizing film squares and lay them side by side on the overhead projector to show students that light is transmitted through them.
2. Lay one film on top of the other and slowly rotate the top film (analyzer) showing the students that light is only passing through the filters in one plane. When the planes of each film are parallel the light passes through. When they are perpendicular the light does not (see Figure 1 in Background).
3. Have students record their observations in the Data Table on the worksheet.
4. Place one piece of plasticware between the two polarizing films.
5. Rotate the analyzer and observe the areas of stress in the plasticware.
6. Have students record their observations in the Data Table on the worksheet.
7. Repeat steps 4–6 for the other plasticware pieces.
8. Place the mica sheet between the two polarizing films.
9. Rotate the analyzer and/or the mica sheet to see the effect of refracted light through the analyzer.
10. Have students record their observations in the Data Table on the worksheet.
11. Place the Iceland spar crystal between the two films.
12. Rotate the analyzer and/or the Iceland spar crystal to see the effects of birefringence through the analyzer.
13. Have students record their observations in the Data Table on the worksheet.
14. Place the flat side of the Iceland spar on a piece of newsprint or text. Allow the students to observe the double image created as a result of birefringence (see Figure 2 in Background).
15. Place the wave retarder between the two films.
16. Rotate the analyzer and/or the wave retarder to see how it affects the path of light through the polarized films.
17. Have students record their observations in the Data Table on the worksheet.

Student Worksheet PDF

12300_Student1.pdf

Teacher Tips

• Benzoic acid crystals are also very interesting to investigate with polarized light. To make benzoic acid crystals, dissolve benzoic acid in acetone in a glass Petri dish and allow the acetone to evaporate. Then, place the Petri dish between two pieces of polarized film on an overhead projector to investigate the effect of crystals on polarized light.
• Using a very inexpensive cellophane tape (Magic Tape™ will not work) or thin packing tape, layer the tape at angles on a microscope slide or over the opening in a 35 mm projector slide. Vary the number of layers and investigate the effect on the polarized light by sandwiching the device between two pieces of polarized film on an overhead projector.
• Obtain various materials such as the window from an envelope, clear plastic deli tray, or a transparency sheet and observe the effects of these materials on polarized light by placing them between two pieces of polarized film on an overhead projector.
• Using a clear plastic protractor or ruler, investigate the effect that these objects have on polarized light. Drill a hole or hit the object with a hammer and observe the stress that is placed on the object by the procedure.
• Students could use this kit in groups of three or four to investigate polarization in a hands-on laboratory experiment.
• The phenomenon of polarization can be demonstrated with a Slinky^® and two textbooks. Have two students each hold a textbook perpendicular to a table or floor and parallel to the other. The books should be slightly farther apart than the width of the slinky. Stretch the slinky between the two books and allow a student to hold each end. Oscillate the slinky up and down and observe that the transverse wave that is created will travel past the books to the other end of the slinky. Now, oscillate the slinky from side to side and notice that the transverse wave does not continue past the books. The textbooks represent the slits in the polarizing film and the slinky represents the transverse light waves.
• Try the slinky demonstration with longitudinal or compression waves to show the students that these waves cannot be polarized.

Sample Data

Answers to Questions

1. What is the definition of polarization?
   

   A process or state in which rays of light exhibit different properties in different directions, especially the state in which all the vibration takes place in one plane.

2. How does an analyzer work?
   

   An analyzer is the second of two polarizing filters in the path of light that is used to determine the orientation of the polarized light from the first polarizing filter. The polarized light from the polarizer is absorbed by the analyzer if it is in a perpendicular orientation and passes through if it is parallel to the first filter.

3. What do optically active materials do to polarized light passing through them?
   

   Optically active materials rotate the plane of polarization relative to the original plane. (The angle of rotation of the plane can be determined by rotating the analyzer until maximum intensity is achieved.)

4. What are examples of optically active materials?
   

   Student answers will vary, but may include mica, Iceland spar or acrylic.

5. Why might cellophane tape, when viewed through an analyzer change color?
   

   The tape changes color when viewed through an analyzer because the cellophane is an optically active material that changes the angle of the plane of polarization as polarized light passes through it.

6. What is birefringence? Give an example.
   

   Birefringence occurs when light is refracted differently at different angles when traveling through a crystal. An example is the double image seen through Iceland spar.

Discussion

One way to test objects without destroying them is to make models of them out of clear plastic and observe them under polarized light. Stresses in the plastic cause it to become deformed, altering its optical properties. The result is the display of colors that occurs where the material is stressed. This is due to changes in the refractive index of the material. The rainbow of colors that is visible in this experiment illustrates the stresses that are placed on the plastic. This is a great way to troubleshoot a potential problem with a large object such as a bridge or building without the risk of damage.

Iceland spar and mica are called “optically active” materials—these are materials that rotate the polarization direction of light passing through it. Iceland spar is a form of optical quality calcite that separates an image into two displaced images that are polarized perpendicular to each other. Most transparent crystalline materials are birefringent; however, this property is exceptionally pronounced with Iceland spar. Mica changes the angle of the plane of polarization by refracting the light. Wave plates or wave retarders are also used to change the index of refraction and the ¼ wave retarder included in this kit may be used to create circularly polarized light.