Birefringent optical elements with two optic axes, one fast and one slow. The incoming beam is separated into mutually orthogonal polarized beams that are recombined with a phase difference that is a function of the thickness of the material and the wavelength of the light. If the phase difference is exactly 1?4 cycle, the plate is called a quarter-wave plate. If the phase difference is exactly 1?2 cycle, the plate is called a half-wave plate.