Editing Waveplate (section)

{{Short description|Optical polarization device}}
[[Image:Waveplate.png|thumb|300px|{{legend|green|Electric field parallel to optic axis}}
{{legend|blue|Electric field perpendicular to axis}}
{{legend|red|The combined field}}
Linearly polarized light entering a half-wave plate can be resolved into two waves, parallel and perpendicular to the optic 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 is a mirror-image of the entry polarization state (relative to the optic axis).]]

A '''waveplate''' or '''retarder''' is an [[optics|optical]] device that alters the [[Polarization (waves)|polarization]] state of a [[light]] wave travelling through it. Two common types of waveplates are the ''half-wave plate'', which rotates the polarization direction of [[linear polarization|linearly polarized]] light, and the ''quarter-wave plate'', which converts between different elliptical polarizations (such as the special case of converting from linearly polarized light to [[circular polarization|circularly polarized]] light and vice versa.)<ref name="hecht">{{cite book|author=Hecht, E.|title=Optics|edition=4th|year=2001|pages=352–5|publisher=Addison-Wesley |isbn=0805385665}}</ref>

Waveplates are constructed out of a [[birefringence|birefringent]] material (such as [[quartz]] or [[mica]], or even plastic), for which the [[index of refraction]] is different for linearly polarized light along one or the other of two certain perpendicular crystal axes. The behavior of a waveplate (that is, whether it is a half-wave plate, a quarter-wave plate, etc.) depends on the thickness of the crystal, the [[wavelength]] of light, and the variation of the index of refraction. By appropriate choice of the relationship between these parameters, it is possible to introduce a controlled phase shift between the two polarization components of a light wave, thereby altering its polarization.<ref name="hecht" /> With an engineered combination of two birefringent materials, an achromatic waveplate<ref>{{Cite web |title=Mounted Achromatic Wave Plates |url=https://www.thorlabs.com/ |access-date=2024-01-16 |website=www.thorlabs.com |language=en}}</ref> can be manufactured such that the spectral response of its phase retardance can be nearly flat.

A common use of waveplates—particularly the sensitive-tint (full-wave) and quarter-wave plates—is in [[optical mineralogy]]. Addition of plates between the [[polarizer]]s of a [[petrographic microscope]] makes the optical identification of [[mineral]]s in [[thin section]]s of [[rock (geology)|rocks]] easier,<ref name=Winchell121/> in particular by allowing deduction of the shape and orientation of the [[optical indicatrix|optical indicatrices]] within the visible crystal sections. This alignment can allow discrimination between minerals which otherwise appear very similar in plane polarized and cross polarized light.