• # question_answer Distinguish between unpolarised and plane polarised light. An unpolarised light is incident on the boundary between two transparent media. State the condition when the reflected wave is totally plane polarised. Find out the expression for the angle of incidence in this case.

Polarisation by reflection from a transparent medium. The reflected, completely plane polarised light makes an angle of $90{}^\circ$ with the direction of corresponding refracted light. Brewster's law According to Brewster, the polarising angle ${{i}_{p}},$ the reflected plane polarised light and refracted rays are perpendicular to each other, then ${{i}_{p}}+r=90{}^\circ$ where, r = angle of refraction. $r=90{}^\circ -{{i}_{p}}$ $\because$Snell's law, $\mu =\frac{\sin {{i}_{p}}}{\sin \,r}=\frac{\sin {{i}_{p}}}{\sin (90{}^\circ -{{i}_{p}})}$             $\mu =\frac{\sin {{i}_{p}}}{\cos {{i}_{p}}}\Rightarrow \mu =\tan {{i}_{p}}$ This relation is known as Brewster's law.