What is a polaroid ? How is plane polarised light obtained with its help ? Mention two devices which use polarised light.

Explanation

Polaroids
Tourmaline crystal is a natural polarising material and is a semi-precious gem and also not available in large size. Polaroids are artificially made large sheets or plates and capable of producing a strong beam of polarised light.
In 1852, W.B. Herpath observed that small needle- shaped crystals of quinine iodosulphate now called Herpathite have the property of polarising light. But the size of these crystals is so small that they cannot be used for large scale polarisation of light. In 1932, E.H. Land invented the technique of aligning the herpathite crystals as long chains parallel to each other. He arranged a large number of these crystals with their axis parallel to one another and placed in between the sheets of plastic. Such sheets act even more efficiently than the tourmaline crystals and are called sheet polarisers or polaroids.
Polaroids can also be obtained from a sheet of polyvinyl by stretching these sheets, the molecules become parallel to the direction of stress. The sheet is impregnated with iodine. This is called H-polaroid. On heating H-polaroids in the presence of a dehydrating agent, it becomes strongly dichoric. This is called k- polaroid.
The polaroid sheets have a characteristic plane called transmission plane. When the axis of these two planes are parallel to one another, the light transmitted by first is also transmitted by second fig. (a), but if the planes are perpendicular to each other, then light transmitted by first is blocked by second fig. (b) and no light comes out.

Uses. Following are two practical uses of polaroids :
1. In sun glasses. Sun glasses fitted with polaroids instead of coloured glasses are more efficient in protecting the eyes from glare due to reflected light without obscuring details even in shadows.
2. Wind shield of automobiles. Wind screen fitted with polaroid as well as head lights of automobiles protect the eyes of the drivers of the automobile from the dazzling light of approaching vehicles. The transmission axes of the polaroids both in the head lights as well as the wind screen are oriented say at `45^(@)` with the vertical.