Selenite wafer between crossed polarizers

Using my laptop's display as a source for polarized light, a thin (~2mm) wafer of selenite was photographed with an analyzer polarizer held in front of the camera's lens.

The left image has the polarizing analyzer perpendicular to the polarization of the laptop's display while the right image has the analyzer parallel to the polarization of the display. The center image has the analyzer at a 45 degree angle to the polarization of the display.

At an angle of 45 degrees, the analyzer is perpendicular to the polarization of either the o-ray or the e-ray passing through the selenite. Because of this that ray is attenuated and there is no resulting interference with the other ray and therefore, no color.

The different colors are due different thickness along the surface of the birefringent selenite. The birefringence of selenite is approximately 0.0100. Therefore the total retardation responsible for the interference (after the analyzer) between the two components (which move at different speeds through the selenite) and which produces a specific color, is the thickness of the selenite x 0.0100.

See this for a more technical explanation:
www.olympusmicro.com/primer/lightandcolor/birefringence.html

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