retarders

A phase retarder is a useful tool for manipulating polarized light by changing the optical path length for one of the planes of polarization.

In general, quarter-wave retarders introduce a phase shift, resulting in circularly polarized light. Half-wave retarders produce a λ/2 phase shift and rotate the polarization of plane-polarized light by twice the angle between the fast axis of the retarder and the plane of polarization.

The passage of circularly polarized light through a half-wave plate changes the direction of rotation of the polarization.