2” x 2” slides that support a small piece of polarizing film. This film blocks all but the vertical wavelengths of light. When you hold two slides together and rotate one of them 90 degrees, all light is blocked. Continue to rotate and the viewing field is clear until you rotate the slide 270 degrees and it is black again. Can be used for other interesting effects.
Demos and Labs: Corn Syrup and Polarizing Light