Nicol prism

Nicol prism at the Mineralogical Collection "Luigi Bombicci Museum" of the University of Bologna, in Bologna
Schematic representation of the propagation of light in a Nicol prism showing the splitting of unpolarized light into ordinary and extraordinary polarized rays

A Nicol prism is a type of polarizer. It is an optical device made from calcite crystal used to convert ordinary light into plane polarized light. It is made in such a way that it eliminates one of the rays by total internal reflection, i.e. the ordinary ray is eliminated and only the extraordinary ray is transmitted through the prism.

It was the first type of polarizing prism, invented in 1828 by William Nicol (1770–1851) of Edinburgh.