The waveplate resolves a linear polarized light wave into two orthogonal, linear polarized components and produces a phase shift between them. 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.

Diameter 0.5 Inch; Zero Order; Quarter-wave or Half-wave; AR Coated