A second way to polarize light uses the double refraction that occurs when light passes through an anisotropic material. Calcite is an anisotropic mineral which is often used for this purpose.
The critical angle and total internal reflection
There is a certain angle of incidence, the angle at which light approaches a surface, which will cause the refracted ray to lie along the interface between two materials. The angle of incidence which gives rise to this situation is called the critical angle. If the angle of incidence is greater than the critical angle, no refraction will occur and all light will be reflected. This is called total internal reflection.
A Nicol prism is a device used for polarization in many older polarizing microscopes. The prism is made using a clear calcite crystal. The crystal is cut on the diagonal and is glued back together using balsam (which has an index of refraction of 1.537). Because calcite is an anisotropic mineral, when monochromatic, unpolarized light enters the crystal, it is split into an e and w ray. The rays have two different velocities, and therefore have two different indices of refraction. The index of refraction of the e ray is lower than that of balsam, while the index of refraction for the w ray is greater than that of balsam. The cut of the crystal is such that when the w ray strikes the interface between the calcite and the balsam its angle of incidence is greater than the critical angle, and it is totally internally reflected and absorbed. The e ray is allowed to enter the balsam and is transmitted as plane polarized light.
Courtesy of Greg Finn, Brock University http://www.brocku.ca/earthsciences/people/gfinn/
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