What is Brewster's angle? When an unpolarized light is incident on a plane glass surface, what should be the angle of incidence so that the reflected and refracted rays are perpendicular to each other?

Brewster's angle:

The partially reflected light gets partially plane polarized when ordinary light, is allowed to undergo refraction. However, there is an angle of incidence at which, an ordinary light undergoes refraction as well as reflection (partial) and then the partially reflected ray is richly plane polarized. Such an angle is known as polarizing angle or Brewster's angle. It is denoted by i_p. 

Light can be polarized by reflecting it from a transparent medium. The extent of polarization depends on the angle of incidence. At a particular angle of incidence, called Brewster's angle, the reflected light is completely polarized as shown in the diagram below: 



When reflected and refracted rays are perpendicular to each other, angle of incidence becomes equal to the polarizing angle. 

                    $\mathrm{i} = {\tan }^{-1} \mathrm{\mu }$ 

where, $\mathrm{\mu }$ is the refractive index of the medium.