How is polarisation of light obtained by scattering of light?

Explanation

(i) The light from a clear blue portion of the sky shows a rise and fall of intensity when viewed through a polaroid which is rotated. (ii) This is because of sunlight, which has changed its direction (having been scattered) on encountering the molecules of the earth's atmosphere. (iii) The incident sunlight is unpolarised. The electric field of light interact with the electrons present in the air molecules. (iv) Under the influence of the electric field of the incident wave the electrons in the molecules acquire components of motion in both these directions. (v) An observer looking at `90^@` to the direction of the sun. Clearly, charges accelerating parallel do not radiate energy towards this observer since their acceleration has no transverse component. (vi) The radiation scattered by the molecule is therefore polarized perpendicular to the plane. This explains the reason for polarisation of sunlight by scattering.