Discuss polarisation by selective absorption.

Explanation

(i) Selective absorption is the property of a material which transmits waves whose electric fields vibrate in a plane parallel to a certain direction of orientation and absorbs all other waves.
(ii) The polaroids or polarisers are thin commercial sheets which make use of the property of selective absorption to produce an intense beam of plane polarised light. Selective absorption is also called as dichroism.
(iii) Tourmaline is a natural polarising material.
(iv) Polaroids are also made artificially.
(v) Small needle shaped crystals of quinine iodosulphate have the property of polarising light.
(vi) A number of these crystals with their axes parallel to one another packed in between two transparent plastic sheets serve as a good polaroid.
(vii) New types of polaroids are prepared in which thin film of polyvinyl alcohol is used. These are colourless crystals which transmit more light, and give better polarisation.