- University of Virginia
- Physics Department
A Physical Science Activity
2003 Virginia SOLs
- describe light as a wave that experiences interference;
- predict the behavior of light through polarizing films.
- 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.
- two polarizing sheets
- overhead projector
- plastic fork
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.
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,
A good description and some activities for polarized light
can be found on the Exploratorium site. Click
here to go to the site.
To print out the Student Copy only,
- 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
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.
- 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.
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.
- 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.
- 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.
- Search for uses of polarizing filters. Examples include sunglasses and
- 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
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.
- Students explain in a paragraph how the experiment with
polarizing lenses demonstrates the transverse wave nature of
- Given schematics of polarizing lenses, students determine
whether light will pass through the filters or not.