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Thermodynamics

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Dichroism

There are certain crystals and minerals which are doubly refracting and have the property of absorbing the ordinary and the extraordinary rays unequally. In this way, plane polarized light is produced. The crystals showing the property are said to be dichroic and the phenomenon is known Dichroism. Tourmaline is a dichroic crystal and absorbs the ordinary ray completely absorbed while the extraordinary ray is partly absorbed and so it emerges.

Herapath (English), in 1852, discovered a synthetic crystalline material iodo-sulphate is quinine known herapathite which is a dichroic crystal. It transmits linearly polarized light of all colours and wavelengths. But these crystals are not stable and are affected by slight strain. In 1934, Land developed a polarizer known as a Polaroid in the form of large sheets.

Herapathite crystals are embedded in a volatile viscous medium and the crystals are aligned with their optic axes parallel. They are prevented from shattering. There are number of methods of preparing Polaroid sheets. In one process, the dichroic crystals are embedded and arranged with their optic axes parallel in cellulose acetate. A more recent type is prepared by taking a sheet of polyvinyl alcohol and subjecting it to a large strain. In this way, the molecules are oriented parallel to the strain and the material becomes doubly refracting. It acts as a dichroic crystal when strained with iodine. The Polaroid sheets are placed between glass plates so crossed, the emergent beam is plane polarized. When the two polaroids are crossed, there is perfect extinction of light.

Uses of polaroids: Polaroids are widely used as polarizing sun glasses. Polaroids films are used to produce three-dimensional moving pictures. They are used to produce three-dimensional moving pictures. They are used to eliminate the head light glare in motor cars. They are also used to improve the colour contrasts in old oil paintings and as glass windows trains and aeroplanes. In aeroplanes, one of the polaroids is fixed while the other can be rotated to control the amount of light coming inside.