Nicolsprism

Nicol's prism is an instrument for polarising light or for analysing it when polarised. It is made by cutting a piece of Iceland spar in a particular diagonal plane and after polishing the cut surfaces sticking the two halves together again by means of Canada balsam. The Canada balsam has such an index of refraction that the ordinary ray is totally reflected, and only the extraordinary ray passes through the prism. To understand the action of a Nicol's prism some knowledge of polarisation and double refraction is needed.