Question

State clearly how an unpolarised light gets linearly polarised when passed through a polaroid.
(i) Unpolarised light of intensity $I_0$ is incident on a polaroid $P_1$ which is kept near another polaroid $P_2$ whose pass axis is parallel to that of $P_1$. How will the intensities of light, $I_1$ and $I_2$, transmitted by the polaroids $P_1$ and $P_2$ respectively, change on rotating $P_1$ without disturbing $P_2$?
(ii) Write the relation between the intensities $I_2$ and $I_1$

Answer 1

Polaroid consists of long chain polymers which allow the vibration of electric field inside them in only one direction. Hence, the output light wave is linearly polarized in a single direction as shown in the attached figure.

(i)

On passing light through a single polaroid, intensity is reduced to half.

The output intensity from Polaroid 1 is given by:

$I_1=\frac{I_o}{2}$

Hence, $I_1$ will not change on rotating $P_1$. However, intensity of $I_2$ will be changed as discussed below.

(ii)

Initially, axis of Polaroid 2 is aligned to that of Polaroid 1 and then, Polaroid 1 is rotated so that angle between them changes to $\theta$.

Hence, from Malus' Law

$I_2=I_1{\cos }^2\left(\theta \right)$

$I_2=\frac{I_o}{2}{\cos }^2\left(\theta \right)$