Light polarizing discs.
Two polarizing discs are placed on top of one another on the overhead projector with their polarization axes at 90 degrees to each other. With this orientation, no light passes through the discs. Then if a third polarizing disc is placed between the first two and if it is rotated to a proper position, light will pass through the polarizers. Note that by placing a third polarizer (which is an absorber) in the system, the intensity of transmitted light increases.
|Three light polarizing discs||021-12A|
|Overhead projector||Overhead on a cart by desk in SW 021|