A wave plate is an optical instrument that alters the polarization state of wave travelling through it. It is basically of 3 types-Half, full, quarter wave plate. In this experiment laser light passes through a wave plate and helps determine the polarization state of light.

A half-wave plate is very handy in rotating the plane of polarization of a polarized laser to any other desired plane. Most large ion lasers are vertically polarized. To obtain horizontal polarization, simply place a half-wave plate in the beam with its fast (or slow) axis 45° to the vertical. The thickness of half wave plate is such that the phase difference is 1/2 wavelength (l/2, Zero order) or certain multiple of 1/2-wavelength [(2n+1)l/2, multiple order].

A Quarter wave plate are used to turn plane-polarized light into circularly and elliptically polarized light. The thickness of the quarter wave plate is such that the phase difference is 1/4 wavelength (l/4, Zero order) or certain multiple of 1/4-wavelength [(2n+1)l/4, multiple order].