Polarization by dichroic crystals.

Dichroism is exhibited by several minerals and by some organic compounds. The best known of the mineral crystals is tourmaline. When the ordinary light is sent through a thin slab of tourmaline crystal, the transmitted light is polarized. We take two crystals T₁ and T₂  which are parallel to each other. The light transmitted by the first crystal is also transmitted by the second crystal. But suppose we rotate the second crystal through 90⁰, no light is transmitted. Because the absorption by tourmaline of all light rays vibrating in one particular plane, not all planes.

Attempts to produce polarizing crystals of large aperture were made by Herapath in 1852. He was successful in producing small crystals of the organic compound quinine iodo sulfate now it is known as herapathite. In 1932 Land invented the Polaroid which has been used in many different kinds of optical instruments. (Land was a scientist who invented Polaroid) Land and Rogers found further that when an oriented transparent film of polyvinyl alcohol is heated in the presence of an active dehydrating catalyst such as hydrogen chloride, the film darkens slightly and becomes strongly dichroic.

Nowadays polarizing films have more usage. Polarizing films are usually mounted between two thin plates of optical glass. These polarizing films are used to increase the wavelength of light rays in some other light-transmitted systems. Like a car headlight, we add polarized films that will help to increase the wavelength of light rays.