Optical wave plates, also known as retarders, are optical elements that alter the polarization state of the light waves that pass through them. Optical wave plates are usually just a birefringent crystal with very specific orientation and thickness and it does it's job by shifting the phase between two perpendicular polarization components of a light wave. The crystal is cut so that the inherent optical axis of the crystal is parallel to the surfaces of the plate. Light polarized along the optical axis travels through the crystal at a different speed than light with the perpendicular polarization, creating a phase difference. Depending on the thickness of the crystal, light with polarization components along both axes will emerge in a different polarization state.
There are a few different types of optical wave plates. Quarter optical wave plates create a quarter wavelength phase shift and transforms linearly polarized light to circularly polarized light and vice versa. Half optical wave plates retard one polarization by half a wavelength (180 degrees) resulting in a rotation in the polarization direction of linearly polarized light.
The phase difference imparted by optical wave plates depends on the wavelength of the light that is passing through the plate so optical wave plates are manufactured to be wavelength specific.
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