A slab that delays one polarization by 1/2 wave with respect to the other is a **half-wave plate**. A circular one rotates polarization by 90 degrees; a linear one flips one diagonal to the other diagonal and exchanges left and right circularly polarized light.

A slab that delays one polarization by 1/4 wave is a *quarter-wave plate*. A circular one rotates by 45 degrees; a linear one converts diagonal light to circular light and circular light to the other diagonal.

Two pieces of dichroic material with different axis directions can be made into a *Wollaston prism*. This takes light of two different polarizations and sends them in different directions. With a quarter-wave plate switch, this is used as the detector in quantum cryptography.