Waveplate
![A half-wave plate. Linearly polarized light entering a waveplate can be resolved into two waves, parallel (shown as green) and perpendicular (blue) to the optical axis of the waveplate. In the plate, the parallel wave propagates slightly slower than the perpendicular one. At the far side of the plate, the parallel wave is exactly half of a wavelength delayed relative to the perpendicular wave, and the resulting combination (red) is a mirror-image of the entry polarization state (relative to the optical axis).](/Images/godic/202501/31/Waveplate4102.png")
![A wave passing through a half-wave plate.](/Images/godic/202501/31/Waveplate_notext4102.png")
![Two waves differing by a quarter-phase shift for one axis.](/Images/godic/202501/31/Circular.Polarization.Circularly.Polarized.Light_And.Linearly.Polarized.Light.Comparison.svg4102.png")
![](/Images/godic/202501/31/Quarter_wave_plate_polarizaton4102.gif")
A waveplate or retarder is an optical device that alters the polarization state of a light wave travelling through it. Two common types of waveplates 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 and vice versa. A quarter wave plate can be used to produce elliptical polarization as well.