retardation plates A plate cut from a birefringent crystal* like quartz or calcite parallel to the optic axis*. A ray of light falling normally on such a plate, splits into two parts, the ordinary ray and the extra ordinary ray. The two rays travel along the same direction but with different velocities. Depending upon the thickness of the plates there will be phase difference between them when the emerge from the plate. The corresponding effective path difference between the rays is given by,

path difference = (m _O- m _E) t

where m _O and m _E are the refractive indices corresponding to the ordinary ray and extra ordinary ray respectively; t is the thickness of the plate. For a half wave plate the path difference is l /2 (phase difference 180° ) and for a quarter wave plate the path difference is l /4 (phase difference 90° ).