In Experiment P57, one uses an incandescent light source, which emits mostly infrared light.
The polarizing material called for in this experiment is a dichroic polymer (Polaroid film), which is very effective at polarizing visible light, but ineffective at polarizing infrared radiation. Since the CI-6504A Light Sensor, PS-2176, and PS-2106A light sensors are sensitive to wavelengths ranging from 320-1100nm, they will detect both the polarized visible light and unpolarized infrared radiation that is transmitted through both polarizers. The ratio between the measured intensity when the polarizer transmission axes are parallel, I(0), and perpendicular to each other, I(90) is usually less than two when using the setup described in Experiment P57.
One solution to this difficulty is to insert an infrared reflector ("hot mirror") between the light source and the first polarizer. This increases the ratio I(0)/I(90) increased to about 7.
The best solution to the difficulties encountered in this experiment is to use a laser, since the range of wavelengths incident on the polarizers is significantly reduced. Replacing the light source with the OS-8525 diode laser, the I(0)/I(90) ~ 180. See EX-9917A Polarization Experiment using a laser for ScienceWorkshop or EX-9958 Polarization Experiment with Lasers for PASPort for newer laser-based versions of this experiment. The output from a laser will generally be linearly polarized; however, the angle of polarization may switch randomly on some laser diodes. After the laser warms up, place a polarizer in front of the laser that you are using and check that the intensity varies predictably as the polarizer is rotated relative to the laser.