(a) How does an unpolarized light incident on a Polaroid get polarized?

Describe briefly, with the help of a necessary diagram, the polarization of light by reflection from a transparent medium.

(b) Two Polaroid’s ‘A’ and ‘B’ are kept in crossed position. How should a third Polaroid ‘C’ be placed between them so that the intensity of polarized light transmitted by Polaroid B reduces to 1/8th of the intensity of unpolarized light incident on A?

a) When an unpolarized light falls on a polaroid, only those electric vectors that are oscillating along a direction perpendicular to the aligned molecules will pass through. Thus, incident light gets linearly polarized.

Electric vectors which are along the direction of the aligned molecules gets absorbed.
 

Whenever unpolarized light is incident on the boundary between two transparent media, the reflected light gets partially or completely polarized. When reflected light is perpendicular to the refracted light, the reflected light is a completely polarized light.

b) Let the angle between A and C be  .
Intensity of light passing through A = I_o/2 
Intensity of light passing through C = I_o/2 cos² 
Intensity of light passing through B = I_o/2 cos² . cos² (90 - )
Given that, 
 

Thus, the third polaroid is placed at an angle of 45^o.