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 $I_1$ and $I_2$.

Answer 1

Polaroid filters are made of a special material that is capable of blocking of the two planes of vibration of an electromagnetic wave. In this sense, a polaroid serves as a device that filter out one-half of the vibrations upon transmission of the light through the filter. When unpolarised light is transmitted through a polaroid filter, it emerge with one-half the intensity and with vibrations in a single plane. Then it emerges as polarised light.


(i) When an unpolarised light of intenisty $I_0$ is incident on a polaroid $P_1$ which is kept near. To another $P_2$ whose pass axis is parallel to that of $P_1$ the light intensity $I_1$ will become half of the incident intensity.

$I_1=\frac{I_0}{2}$

and when it will pass through $P_2$ polaroid there is no change in the intensity but the angle will change by $co{s}^2\theta$

$I_2=I_{1}co{s}^2\theta =\frac{I_0}{2}co{s}^2\theta$

(ii) The relation between $I_1$ and $I_2$ is given by

$\frac{I_2}{I_1}=co{s}^2\theta$