Ordinary light can be polarised by:
(i) Polarisation of Light by Refraction: When ordinary (unpolarised) light is incident on a parallel-faced glass-plate at the polarising angle, then the light reflected from both the surfaces of the plate is 'completely' plane-polarised. Hence, if a number of such plates are I one above the other and unpolarised light is allowed to fall on the uppermost plate at the polarising angle (fig.), then the component of light whose vibrations are placed perpendicular to the plane of incidence is reflected at each surface of the plate. Since the are mutually parallel, the same happens at all the plates. If the number of plates is quite large, then the emergent light contains mainly that component whose vibrations are plates parallel to the plane of incidence. Thus, the emergent light is also almost plane-polarised. This arrangement is called a 'pile of plates'.
(ii) Polarisation of Light by Double Refraction: There are certain crystals like calcite, quartz, tourmaline, etc., such that when a ray of light is incident upon them, then, on entering the crystal, the ray is splitted into two refracted rays. This phenomenon is called 'double-refraction'. One of the refracted rays obeys the laws of refraction and is called the 'ordinary ray'. The other refracted ray does not obey these laws; hence it is called the 'extraordinary ray'. Both these rays are plane-polarised in mutually perpendicular planes. Thus, by double refraction we obtain polarised light.
