A. Alan Middleton and Samuel Sampere, Syracuse University, Department of Physics, Syracuse, NY 13244; aam@syr.edu; sampere@phy.syr.edu. (August 8, 2000)

The study of light is, of course, a powerful and central topic in physics. This subject also lends itself so very easily to teaching. Students can create numerous entrancing visual effects using simple instruments (such as diffraction patterns using aluminum foil pinholes) and evocative demonstrations are abundant.^1,2 Applications of ray and wave theories of light, ranging from vision to CD-ROMs to liquid crystal displays (LCDs), provide strong motivation.³ We describe here a demonstration exercise that we have found useful for encouraging students to think about some of the concepts of light that they have studied, namely bringing together concepts from color theory and the polarization of electromagnetic radiation.

The investigation of vision and light occupies most of the first quarter of the two-semester sequence "Science for the 21^st Century" at our university. Almost all of the students are non-science majors and conceptual understanding is emphasized. Experiments with light are useful in building semi-qualitative reasoning skills. The first semester focuses on basic physical ideas that underlie modern technology, in particular, the wide range of technologies that are used in a laptop computer. In order to understand laptop displays, one needs to understand color (trichromacy) theory, liquid crystals, and polarization of light.

Part of the graded work for the course are "demo challenges".⁴ These exercises ask the students to apply the knowledge they have gained in the lab that week. An apparatus is presented to the students and the instructor answers students' questions about the setup. The students are then asked to predict the outcome of an experiment involving that apparatus. They keep their lab work as a resource for considering the problem and hand in their prediction with their lab at the end of the week, just before the experiment is performed in a lecture meeting.

The first lab of the semester is a lab on color mixing using a light box. Later in the semester, the students gain experience with polarizing filters. We ask them to observe light reflected off of glass and trees through a single polarizer, the intensity of light that passes through a pair of polarizers at various angles, and the effect of placing a third polarizer between a pair of crossed polarizers. The challenge that we describe here is presented to the students just after they have completed the latter lab.

The central components of the apparatus are two light sources (we use red and green lasers, though two flashlights with color filters also work with a white screen), three polarizing filters, and a "screen". A representative apparatus arrangement is shown in Fig. 1. For laser illumination, the "screen" is a Ping-Pong ball that has a 1 cm hole pierced in it. The laser light entering the hole is then scattered within the ball, illuminating it approximately uniformly. Two of the polarizing filters are placed at perpendicular orientations, one in the path of each of the beams. The third filter is placed just in front of the ball. This filter can be rotated about an axis perpendicular to the long axis of the setup.

The question we ask the students is "How will the perceived color of the ball change, if at all, as the third filter is rotated?" The answer is that when the axis of the third filter is aligned with that of the polarizer for the red beam, the ball will appear red. When the axis is aligned with that of the polarizer for the green beam, the ball will appear green. At intermediate angles, one obtains a range of perceived colors including orange and yellow. The intensity of red light that illuminates the ball varies from approximately zero to a maximum as the intensity of green light correspondingly diminishes from its maximum to nearly zero. This can be seen by considering each beam individually and can be demonstrated by turning on one laser at a time. This range of colors can then be understood using trichromacy theory.

This apparatus exhibits two important concepts in light and vision. It is also a very pretty "demonstration" to watch (see Fig. 2.) For smaller classes, the students can gather around (taking care to keep people out of the path of stray laser light), while for large classes, a video camera connected to our projection system works quite well. The room is darkened during the demonstration.

We find in practice that the average credit for this challenge is about 50%. Most of the students correctly predict the red and green ends of the range, while slightly less than half include a correct description of why one sees yellow light.

Photos of the apparatus, a description of a related apparatus that uses light polarized by reflection, and an example of the demo challenge question are available at http://www.phy.syr.edu/courses/demos.

1. R. Ehrlich, Turning the World Inside Out, (Princeton University Press, 1990).
2. B. Franklin, Teaching about Color and Color Vision, (American Association of Physics Teachers, 1996).
3. L. A. Bloomfield, How Things Work: The Physics of Everyday Life, (John Wiley & Sons, 1997).
4. D. Duncan, "A Challenging Way to Teach Introductory Astronomy", 1998 ASP Symposium On Teaching Astronomy to Non-Science Majors.