The retardation plate or the phase shifter is an optical component which can rotate or modify the plane of polarization of the incident beam falling on to it. In its simplest form, the retardation plate consists of a uniaxial crystal cut in such a way that the the optics axis or symmetry axis lies in the plane of the plate. If a retardation plates introduces a phase change of or (p being an integer) in the two mutually orthogonal components of electric field associated with the light wave then it is called as half wave plate . As in such case the relative path difference introduced in the two components is or half the wavelength. If the corresponding phase difference introduced is or , then it is called as quarter wave plate as in this situation the relative path difference introduced