A retarder or wave plate is an optical device that alters the polarization state of a light wave travelling through it. Two common types of wave plates 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.
Retarder plates or wave plates are constructed out of a birefringent material, such as quartz or mica, for which the index of refraction is different for different orientations of light passing through it. The behavior of a wave plate depends on the thickness of the crystal, the wavelength of light, and the variation of the index of refraction. Thus it is possible to control the phase shift between the two polarization components of light wave by varying these three components.
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