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Waveplates

A Waveplate is an optical element having two principal axes, slow and fast, that resolve an incident polarized beam into two mutually perpendicular polarized beams. The emerging beam re-combines to form a particular single polarized beam. Wave plates produce full-, half- and quarter-waves of retardation. They are also known as a retardation plate. (Ref: The Photonics Dictionary)

Quarter Wave Plate
When linearly polarized light is input at 45 degrees to the axis of a quarter waveplate, the output is circularly polarized, and vice versa.

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Half Waveplate
A half waveplate rotates linearly polarized light to any desired orientation. The rotation angle is twice the angle between the incident polarized light and optical axis.