A wave plate is an optical device that can generate an additional optical path difference (or phase difference)
between two mutually perpendicular light vibrations. Usually made of quartz, MgF2, etc. wafers with precise thicknesses whose optical axis is parallel to the wafer surface. The linearly polarized light is vertically incident on the wafer, and its vibration direction is at an angle θ between the optical axis of the wafer (θ≠0, ), and the incident light vibration is decomposed into two parts: perpendicular to the optical axis (o vibration) and parallel to the optical axis (e vibration). components, which correspond to the o light and the e light in the wafer (see birefringence)。