A Polaroid is a material which polarizes light. The phenomenon of selective absorption is made use of in the construction of polariods.
There are different types of polaroids.
A Polaroid consists of micro crystals of herapathite (an iodosulphate of quinine). Each crystal is a doubly refracting medium, which absorbs the ordinary ray and transmits only the extra ordinary ray. The modern polaroid consists of a large number of ultra microscopic crystals of herapathite embedded with their optic axes, parallel, in a matrix of nitro –cellulose.
Recently, new types of polariod are prepared in which thin film of polyvinyl alcohol is used. These are colorless crystals which transmit more light, and give better polarization.