Optics component. A carefully adjusted thickness of birefringent material such that the light associated with the larger index of refraction is retarded by some amount in phase with respect to that associated with the smaller index. The material is cut so that the optic axis is parallel to the front and back plates of the plate. Any linearly polarized light which strikes the plate will be divided into two components with different indices of refraction.
For instance a Quarter Wave Plate will create a quarter wavelength (90°) phase shift and change linearly polarized light to circular and vice versa. This is done by adjusting the plane of the incident light so that it makes 45° angle with the optical axis. This gives equal amplitude ordinary and extra-ordinary waves. When the ordinary wave is slower, as in calcite, the ordinary wave will fall behind by 90° in phase, producing circularly polarized light.