A wave plate is an optical device that produces an additional optical path difference (or phase difference) between two perpendicular optical vibrations. Usually made of a birefringent wafer of precise thickness, such as quartz, calcite, or mica, with an optical axis parallel to the surface of the wafer. When linearly polarized light is vertically incident on the wafer, its vibration direction is sandwiched with θ Angle (θ≠0,) between the optical axis of the wafer, and the incident optical vibration is decomposed into two components perpendicular to the optical axis (o vibration) and parallel to the optical axis (e vibration), which correspond to the o light and e light in the wafer (see birefringence).