Editing Waveplate (section)

== Use in mineralogy and optical petrology ==
[[File:Caesium Chloride.jpg|thumb|Thin crystalline film of [[caesium chloride]] photographed using a petrographic microscope.]]
The sensitive-tint (full-wave) and quarter-wave plates are widely used in the field of [[optical mineralogy]]. Addition of plates between the polarizers of a [[petrographic microscope]] makes easier the optical identification of [[mineral]]s in [[thin section]]s of [[rock (geology)|rocks]],<ref name=Winchell121/> in particular by allowing deduction of the shape and orientation of the [[optical indicatrix|optical indicatrices]] within the visible crystal sections.

In practical terms, the plate is inserted between the perpendicular polarizers at an angle of 45 degrees. This allows two different procedures to be carried out to investigate the mineral under the crosshairs of the microscope. Firstly, in ordinary cross polarized light, the plate can be used to distinguish the orientation of the optical [[index ellipsoid|indicatrix]] relative to crystal elongation – that is, whether the mineral is "length slow" or "length fast" – based on whether the visible interference colors increase or decrease by one order when the plate is added. Secondly, a slightly more complex procedure allows for a tint plate to be used in conjunction with [[interference figure]] techniques to allow measurement of the ''optic angle'' of the mineral. The optic angle (often notated as "2V") can both be diagnostic of mineral type, as well as in some cases revealing information about the variation of chemical composition within a single mineral type.