A wave plate or retarder is an optical device that alters the polarization state of a light wave travelling through it. A wave plate works by shifting the phase of the light wave between two perpendicular polarization components. A typical wave plate is simply a birefringent crystal with a carefully chosen thickness. The crystal is cut so that the extraordinary axis (polarized parallel to the axis of anisotropy) is parallel to the surfaces of the plate. When the extraordinary index is smaller than the ordinary index (polarized perpendicularly to axis of anisotropy), 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