Fun with Polarizers BY MICHAEL E. KNOTTS

AND JENNIFER M. RICE

Next time you go on the road to do a community outreach pro­

gram in optics, don't forget to bring polarizers. Coupled with an over­head projector or a light box, they make terrific tools for demonstrat­ing properties of light from birefrin­gence to Rayleigh scattering.

Dichroic sheet polarizers The invention of dichroic sheet polarizers by Edwin Land represents a fascinating chapter in 20th century science. Land perfected a process for mass-producing polarizers by using a mechanical stretching method to orient microscopic polarizing crys­tals in a cellulose acetate binder. This technology was further refined to include various schemes of dop­ing oriented polymer molecules

(e.g., iodine and polyvinyl alcohol). An excellent source on the history of dichroic polarizers and many other topics, check out Edwin H. Land's Essays, Volume I: Polarizers and Instant Photography edited by Mary McCann. 1

The Polaroid Corp. founded by Land still reigns supreme in the manufacture of sheet polarizers, but other companies produce them as well. You can purchase demon­stration material from Edmund Scientific in the form of single large sheets.2 These polarizers transmit about 37% of unpolarized light and have a polarization efficiency of about 99%. But be careful when cutting the large sheet in two. The best approach is to protect the polarizer with paper or a thin plas­tic sheet and cut it with a sheet metal shear, which can be found in most well-equipped general pur­pose machine shops. Alternately, you can carefully cut the material with a sharp pen knife or utility knife. Make multiple scribe marks with a pen knife until the plastic is deeply scored. Then turn the sheet over and carefully scribe directly above the previous marks. Finally bend the sheet to crack it cleanly in two.

Experiments Birefringence Now that you have your polarizers, it's time to take the show on the road. Most kids are impressed when you demonstrate that two polarizers together can extinguish transmitted light as they are crossed. For older children you can go into some detail about the law of Malus and the sinu­soidal variation in the transmitted intensity that occurs.

But kids are visual creatures, and they will likely want to see colorful displays. Birefringence comes to the rescue! Birefringent effects with polarizers elicit "ooohs" and "aaahs" from the audience. Start off by putting one polarizer on the projec­tor and invite a child to hold anoth­er polarizer above the projector. Take strips of a polyethylene sheet (you can just cut up a plastic sand­wich bag) and hold it between the two polarizers (see Fig. 1). Start stretching the strip to show beauti­ful stress patterns. You could explain to the kids that light slows down as it passes through a substance, and the amount it slows down is mea­sured by the index of refraction. Then explain that some solids have different indices of refraction for different polarizations and that this

Figure 1. Stress birefringence in stretched polyethylene plastic sheet viewed with two crossed polarizers.

Figure 2. Stress birefringence in molded styrene cassette tape box viewed with two crossed polarizers.

64 Optics & Photonics News/May 1999

property can change the polariza­tion of the transmitted light by introducing a phase shift between different polarization components. Furthermore, stress in a material can change the magnitude of this effect. Also, the phase shift induced depends on the wavelength, which can result in beautiful, intense color patterns. [For this experiment you can also use a cassette tape holder (see Fig. 2).]

Another great material to use is calcite, which is famous for its bire­fringence. High quality calcite of the type used to make precision polariz­ers is expensive, but small pieces of lower quality suitable for demon­stration experiments can be pur­chased for a few dollars at stores sell­ing gems and minerals. Explain to the children that light entering the calcite crystal is refracted into two separate beams. Depending on the age of the audience you may or may not decide to explain that one beam is polarized perpendicular to the optic axis of the crystal (and obeys Snell's law) and that the other beam is polarized parallel to the plane con­taining the optic axis. No matter what the age of the class you will impress them as you put a trans­parency on the overhead projector, and then put the calcite on top. As you turn the crystal you will see a nice double image. Then, hold a polarizer above the crystal and rotate it to select between the two images.

You can use a bottle of Karo syrup (corn syrup) for a nice demonstration of optical activity (see Fig. 3). Optically active materi­als, such as sugar solutions, rotate the direction of polarization as light propagates through them. To demonstrate this, take the label off a sealed bottle of Karo syrup and place it on top of a polarizer placed on the overhead projector. Hold another polarizer above the bottle and rotate it. You'll see the color of the bottle change as you select dif­

ferent transmitted polar­ization angles (the degree of optical rotation is wavelength dependent).

Rayleigh scattering and Brewster effects Have the children take turns looking at the sky 90 degrees from the sun through a polarizer. Ob­serve that the Rayleigh scattered sunlight is sig­nificantly polarized (never look directly at the sun, even through a pair of crossed polarizers). To observe the same effect in a controlled setting, add a few drops of milk to an aquarium full of water, shine a strong beam of light through the aquari­um, and observe the scat­tered light from the side using a polarizer. Explain that reflections off of car windows, floors, pave­ments, etc., will also be polarized when the angle of reflection approaches the Brewster angle. Ask kids how polarizers are useful in photography.

Polarizers allow some exciting optical phe­nomena to be readily observed. The sheet polarizer is a magnificent invention that no optics road kit should be without.

References 1. Edwin H. Land's Essays, Volume 1:

Polarizers and Instant Photography, Mary McCann, ed. (IS&T, Springfield, VA, 1993).

2. Edmund Scientific (609/573-6250) sells sheet polarizers. A 17" x 39" sheet costs $69; a 17" X 19.5" sheet costs $37; an 8.5" X 19.5" sheet costs $20.40.

OPN Contributing Editor Michael E. Knotts (michael.knotts@gtri.gatech.edu) is a research scien­tist at the Georgia Tech Research Institute, Signature Technology Laboratory, Atlanta, GA. Jennifer Rice (jrice@osa.org) is OPN's senior writer.

Figure 3. An opt ical ly act ive mater ia l (corn syrup) v iewed between polar izers with two different or ienta t ions .

Education outreach Continued from page 63

to the OSA Web site and will include a list of members (OSA members will volunteer to be a resource via e-mail) who are willing to answer questions from teachers, students, and parents about optics and gener­al science. Your involvement in pri­mary and secondary education is imperative to help further the mis­sion of OSA.

Aimée Gibbons (agibbo@osa.org) is member and edu­cation services coordinator in the Communications Dept. at OSA. For more information on how to get Involved, contact Aimée at 202/416-1431.

Optics & Photonics News/May 1999 65