Waveplates and retardation plates are optical elements with two principal axes, one slow and one fast, which resolve an incident polarized beam into two mutually perpendicular polarized beams. Their operation is based on birefringent linear effect, which is the difference in the refractive indices for the beams with parallel and normal polarization towards the optical axis of the crystalline quartz material being within the waveplate plane. The emerging beam recombines to form a particular single polarized beam.

The most common types of waveplates are quarter-wave plaste(lambda/4 plates) and half-wave plales(lambda/2), where the difference of phase delays between the two linear polarization directions is

π/2 or π, respectively, corresponding to propagation phase shifts over a distance of lambda/4 or lambda/2.