A
waveplate or
retarder is an
optical device that alters the
polarization state of a
light wave travelling through it. Two common types of waveplates are the
half-wave plate, which shifts the polarization direction of
linearly polarized light, and the
quarter-wave plate, which converts linearly polarized light into
circularly polarized light and vice versa. A quarter wave plate can be used to produce elliptical polarization as well.