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A waveplate is a type of optical device that changes the polarization of electromagnetic radiation (EMR) passing through. A particular waveplate functions over a specific range of wavelengths. They are used in some instruments to detect specific kinds of polarization or to use their effect (which varies by wavelength) to assist in detecting specific wavelengths. They are constructed of birefringent material, i.e., material that has different refractive indexes for EMR with different polarization and/or direction through the material. Quartz is an example birefringent material.
A half-wave plate (HWP) shifts linear polarization and a quarter-wave plate converts between linear and circular polarization. Such a fraction is exact only for a specific wavelength, but the waveplates are generally used for bands that do not stray far from it. Full-wave plates are used to help detect its specific wavelength: linearly-polarized light of its specific wavelength will exit the same as it entered, but other wavelengths will have their polarization altered.