Wave Plate -- from Eric Weisstein's World of Physics

A half-wave plate introduces a phase difference of radians between perpendicular axes. Rotating a half wave plate by an angle relative to the polarization direction of linearly polarized light will therefore shift its polarization angle by . This can be observed by placing a half-wave plate at a 45° angle between crossed polaroids, and noting that light is transmitted.

A quarter-wave plate introduces a phase difference of radians between perpendicular axes. If a quarter-wave plate is placed at a 45° angle to linearly polarized light, it will be converted to circular. Prisms which can serve as quarter-wave plates include the Fresnel rhomb and Mooney rhomb.

Compensator, Kerr Cell, Kerr Effect