Polarizer Retarder Demo
The above video is a demonstration of light polarization and the Malus Law. Rotating a single polarizer does not change the intensity of the unpolarized room light that is transmitted through the polarizer. With two polarizers in series the transmitted light fraction varies between 0 and 1 as either polarizer is rotated. According to the Malus Law this fraction varies as cos(theta)^2, where theta is the angle between the two transmission axes.
When theta = 90 degrees the polarizer axes are crossed and no light is transmitted. However, the video also shows how the transmission can be restored by placing a clear film retarder (a “half-wave plate”) between the polarizers, as it rotates the incoming polarization so that the angle theta is no longer 90 degrees.