A transparent plate of a birefringent material, such as quartz, cut parallel to the optic axis. Light falling on the plate at 90° to the optic axis is split into an ordinary ray and an extraordinary ray (see double refraction), which travel through the plate at different speeds. By cutting the plate to different thicknesses a specific phase difference can be introduced between the transmitted rays. In the half-wave plate a phase difference of π‎ radians, equivalent to a path difference of half a wavelength, is introduced. In the quarter-wave plate the waves are out of step by one quarter of a wavelength.