A Nicol prism
is a type of polarizer
, an optical device used to generate a beam of polarized light
. It was the first type of polarizing prism
to be invented, in 1828 by William Nicol
(1770–1851) of Edinburgh
. It consists of a rhombohedral
crystal of Iceland spar
(a variety of calcite
) that has been cut at a 68° angle, split diagonally, and then joined again using Canada balsam
(a transparent liquid).
Unpolarized light enters the principal section of the crystal and is split into two polarized rays by birefringence
. One of these rays (the ordinary
-ray) experiences a refractive index
<sub>o</sub> = 1.658 and at the balsam layer (refractive index n
= 1.55) undergoes total internal reflection
at the interface, and is reflected to the side of the prism. The other ray (the extraordinary
-ray) experiences a lower refractive index (n
<sub>e</sub> = 1.486), and is not reflected at the interface, leaving through the second half of the prism
as plane polarized light.
This happens because the angle with which the incident ray is falling on the prism is critical angle of the prism and after refraction the O-Ray is only subjected to total internal reflection since there is a deflection of the ray by 14 degrees.
Nicol prisms were once widely used in microscopy
, and the term "crossed Nicols" (abbreviated as XN
) is still used to refer to observation of a sample between orthogonally orientated polarizers. In most instruments,... Read More