A waveplate or retarder is an optical device that alters the polarization state of a light wave traveling through it.

A waveplate works by shifting the phase of the light wave between two perpendicular polarization components. A typical waveplate is simply a birefringent crystal with a carefully chosen thickness. The crystal is cut so that the extraordinary axis is parallel to the surfaces of the plate. When the extraordinary index is smaller than the ordinary index, as in calcite, the extraordinary axis is called the fast axis and the ordinary axis is called the slow axis. Light polarized along the fast axis propagates faster than light polarized along the slow axis. Thus, depending on the thickness of the crystal, light with polarization components along both axes will emerge in a different polarization state.