Nicol Prism

William Nicol (1828) designed the Nicol prism, an optical device used both to produce and to analyse plane polarised light, exploiting the double refraction of a calcite crystal. A calcite crystal with length three times its breadth is cut, along the diagonal of its principal-section rectangle $ABCD$, into two halves whose face angles are $72°$ and $108°$, then cemented back together with a thin layer of Canada balsam, a transparent cement whose refractive index (1.523) lies between the crystal's ordinary-ray index (1.658) and extraordinary-ray index (1.486). An unpolarised ray of monochromatic light (e.g. from a sodium vapour lamp), entering through face $AC$, splits inside the crystal into an ordinary ray and an extraordinary ray, travelling at different speeds. The ordinary ray strikes the Canada balsam layer at an angle exceeding its own critical angle there, and is totally internally reflected sideways, out of the emergent beam; the extraordinary ray, being below its own (different) critical-angle condition at the same balsam layer, passes straight through and is transmitted, emerging as the sole, plane-polarised output beam. A Nicol prism can serve either as a polarizer or (used to test an incoming beam) as an analyser, but has drawbacks: its cost is high (large, flawless calcite crystals …