Question

(a) What is linearly polarised light. Describe briefly using a diagram how sunlight is polarised.
(b) Unpolarised light is incident on a polaroid. How would the intensity of transmitted light change when the polaroid is rotated?

Answer 1

(a) The light in which the vibrations of electric vector are restricted to a particular plane, is called plane polarised light.
The incident sunlight is unpolarised. The dot and double arrows show the polarisation in the perpendicular and in the plane of the figure respectively. Under the influence of the electric field of the incident wave, the electrons in the molecules of the atmosphere acquire components of motion in both these directions. For an observer looking at ${90}^{\circ }$ to the direction of the sun, the charges accelerating parallel to the double arrows do not radiate energy towards this observer since their acceleration has no transverse component. The radiation scattered by the molecule is therefore, represented by dots. It is linearly polarised perpendicular to the plane of the figure.

(b) If the unpolarised light is incident on a Polaroid, the intensity is reduced by half. Even if the polaroid, the intensity is reduced by half. Even if the polaroid is rotated by angle $\theta$ the average over $co{s}^2\theta =\frac{1}{2}.$ Thus from Malus' law : $I=I_{0}co{s}^2\theta$
$|I|=|I_{0}co{s}^2\theta |$
$=I_0|co{s}^2\theta |=\frac{I_0}{2}$
Thus, the intensity of the transmitted light remains uncharged when the polaroid is rotated.