A waveplate is an optical device that resolves a linear polarized Lightwave into two orthogonal, linear polarized components and produces a phase shift between them.
A waveplate introduces a phase shift between components of polarized light transmitted through the plate. It functions in an optical System to modify and control the relative phase of constituent beams. A waveplate is also referred to as a linear phase retarder, or as a retarder plate. In principle, the retarder or wave plate causes the phase of one of the constituent coherent Polarization states to lag behind the other by a predetermined amount. An optical waveplate introduces a delay in propagation for light polarized in one direction relative to light polarized in an orthogonal direction.