University of Virginia
Physics Department

## Polarized Light

A Physical Science Activity

2003 Virginia SOLs

• PS.1
• PS.9

Objectives

Students will

• describe light as a wave that experiences interference;
• predict the behavior of light through polarizing films.

Motivation for Learning

Driving Questions

What does the word "polarize" mean? In its usage in light and optics it means to affect light or other electromagnetic radiation in such a way that the vibrations assume a definite form. In ordinary light the vibrations are in all directions perpendicular to the path of motion of the light ray. In polarized light the vibrations can be in certain directions. Wave motion of light is a complex subject, because we can't see what is actually waving (or vibrating). It is actually the electric and magnetic fields that are waving, but these are difficult subjects also. At best we can give some simple indications of wave motion even though they don't apply so well to light and other electromagnetic radiation. Most wave motion that we can observe in nature is wave motion of matter (water waves, ocean waves, waves in a rope or spring), but light is different. It doesn't even require a medium. Light can travel in a vacuum.

We can see the effects of the polarization of light by looking at polarized glasses. They have the greatest application of polarization.

Examining Stress

Materials:

• two polarizing sheets
• plastic fork

Procedure:

Place one large polarized sheet on an overhead transparency projector. Place another polarized sheet on top of the first and rotate the top sheet until little or no light comes through from the projector onto the screen. Leave the polarized sheets in this configuration, because the polarizing angles are perpendicular to each other.

Now place the plastic spoon between the two sheets and squeeze the plastic fork in various places. For example, squeeze the tines together or bend the spoon. You should see color variations on the screen due to the effects of the polarized light. In this manner you can tell where the stress is on the plastic. This should also work if you have almost any kind of plastic on which you can put some stress. An L shaped object is best. Polarized light is used on models of objects to tell where the stresses are before the object is constructed.

Background Information

Light travels in transverse waveforms. A transverse wave is one in which the oscillations move at 90° from the direction of propagation of the wave (Figure 1). Transverse waves can be easily demonstrated using a rope, slinky, or long spring. Stretch the slinky on the floor or a table between two students. Have one student hold one end stationary while the other student moves the other end back and forth-thus forming waves along the slinky. As the speed with which the student moves the slinky changes, so do the frequencies (and thus wavelengths) of the waves. This transverse wave motion of light is not clearly seen normally, but experiments on light illustrate this characteristic.

One experiment to demonstrate the wave nature of light uses polarizing filters. Polarizing filters will block light waves that are not oscillating in the same plane as the polarized film's orientation (Figure 2a-b). When light is emitted from most common sources, such as an incandescent or fluorescent bulb, the light is unpolarized-that is, the light waves are vibrating in all directions. A single polarizing filter will block the light not vibrating in the polarizing direction, but the light that is vibrating along the polarizing direction will be transmitted. Adding a second polarizing filter oriented 90° from the first will block the remaining light. You can obtain a sheet of polarizing film (that can be cut into smaller pieces) from Frey Scientific (Low Cost Polarizing Film 6"x6", Catalog #S990228 (year 1998-99) for \$15.40; 100 Paragon Parkway, Mansfield, OH 44903, 888-222-1332). It is also availble at Cenco Physics (Polarizing Films 15cm x 15cm, Catalog #WLS-70948-A (year 2000-2001) for \$14.18; 30cm x 30cm, Catalog #WLS70948-B for \$64.77; P.O.Box 5229, Buffalo Grove, IL 60089-5229, 800-727-4368)

A good description and some activities for polarized light can be found on the Exploratorium site. Click here to go to the site.

### Student Activity

Materials

• Long coiled spring or heavy, thick rope (about 8 ft; maybe 1/2" diameter but it needs to be flexible, not stiff)
• Picket fence (can be constructed from scraps of wood or even cut from a box) (Figure 3)
• Polarizing lenses (polarized sunglass lenses)
• Unpolarized light source (any incandescent or fluorescent bulb or sunlight)

Procedure

The first part of this procedure is best completed as a class demonstration with group discussion. The second part can be accomplished in either large or small groups.

1. Run the rope or coiled spring through the section of fencing. Have two students stretch the rope between them while a third student holds the section of fencing between them so the slats are vertical. The students holding the rope will shake the rope up and down so the oscillations are in the same direction as the slats. The wave will pass through the slats unhindered.
2. The student holding the fence section should rotate it 90° so the slats are horizontal. The wave with up and down oscillations will not travel through the slats when they are in this orientation.

Figure 3. Two students are making a vertical wave with a coiled spring through the picket fence. The long spring can oscillate in the vertical direction, but can generally not wave in the horizontal direction except for certain modes. The wave motion in this case is called transverse, because the motion of the wave (up and down) is perpendicular to the propagation (direction of motion) of the wave. Electromagnetic waves are also transverse.

3. Hold a piece of polarizing lens up to a light source so the light is viewed through it. Rotate the lens and notice that the intensity of the light does not appear to change.
4. Hold two pieces of polarizing lens up to a light source. Hold one piece stationary but rotate the other like the piece of fence section. Notice the intensity of the light changes as this second filter is rotated. When the lenses are at right angles, no light is transmitted through the pair of lenses. When the lenses are parallel, however, the same amount of light is transmitted, as though only one lens were present.

Extensions

1. Search for uses of polarizing filters. Examples include sunglasses and three-dimensional movies.
2. Obtain a piece of plexiglas cut out in an "L" shape like in a carpenter's square (but not so big) and place it on top of a transparency projector with a piece of polaroid underneath the plexiglas. Then bend the plexiglas somewhat by pulling apart the two ends. By placing polaroids under and over the plexiglas, you will be able to observe the stress in the plexiglas. The stressing changes the polarizing characteristics of light passing through the plexiglas.

Students with Special Needs

Students with limited visual capacity can still experience the polarization demonstration with the rope and fence section by holding the stationary end of the rope and feeling the difference in the waves that reach them through the different orientations of the fence section.

Click here for further information on laboratories with students with special needs.

Assessment

1. Students explain in a paragraph how the experiment with polarizing lenses demonstrates the transverse wave nature of light.
2. Given schematics of polarizing lenses, students determine whether light will pass through the filters or not.