Question

(a) Distinguish between unpolarized light and linearly polarized light. How does one get linearly polarised light with the help of a plaroid ?
(b) A narrow beam of unpolarised light of intensity `I_(0)` is incident on a polaroid `P_(1)`. The light transmitted by it is then incident on a second polaroid `P_(2)` with its pass axis making angle of `60^(@)` relative to the pass axis of `P_(1)`. Find the intensity of the light transmitted by `P_(2)`.

Answer 1

A light wave in which electric vector oscillates in all possible directions in a plane perpendicular to the direction of propagation is unpolarised light.

If the electric vector vibrationis restricatedto one direction only in a plane perpendicular to the direction of propagation the light is linearly polarised.

● In a polaroid a long chaing of molecules is aligned in a particular direction. The electric vectors (of light waves) along the direction of aligned molecules gets absorbed. An unpolarised light wave incident on such a polaroid gets absorbed. An unpolaried light wave incident on such a polaroid gets linearly plaroised with an electric vector oscillating along a direction perpendicular to the aligned molecules. This direction is called the pass-axis of the polaroid.
(b) After passing from `P_(1)`, the intensity of the light will be `(I_(0))/(2)` .
Now, after passing through `P_(2)`, th new intensity of the light will be `(I_(0))/(2) cos^(2) theta`.
`rArr I = (I_(0))/(2) cos^(2) 60^(@)`.
`rArr I = (I_(0))/(2) cos^(2) 60^(@)`.
`I = (I_(0))/(2) xx I/4 = (I_(0))/(8)`.