Explain how an unpolarised light gets polarised when incident on the interface separating the two transparent media.

In an unpolarised light libration are symmetric about the direction of propagation. For an unpolarised wave, the displacement will be randomly changing with time, though it will always be perpendicular to the direction of propagation. When unpolarised light is incident on the boundary between two transparent media, the reflected light is polarised with its elastic vector parallel to the plane of incidence when the refracted and reflected rays make a right angle with each other. Since we have iB + r = π/2 (Reflected wave in totally polarised) [iB = between angle] M = sin iB / sin r = sin iB / sin (π/2 - iB) = tan iB