(a) 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. Describe, with the help of a suitable diagram, the basic phenomenon/process which occurs to explain this observation.

(b) Show how light reflected from a transparent medium gets polarized. Hence deduce Brewster’s law.

(a) Light emitted by the sun is unpolarised light. The electric field of this incident wave interacts with the electrons in the molecules of air, thus giving them motion in the directions of polarization. Now, if the observer’s plane is to the direction of sun, the charges accelerating parallel to the arrows have no transverse component. Thus they won’t radiate energy as they move towards the observer.

The radiation scattered by the molecule after polarization is given as dots. As the polaroid is rotated, depending on the angle we see the rise and fall of intensity of light.

(b) The following diagram shows that at a particular angle, the reflected ray bounced off a refractive surface is completely polarized.

When the unpolarised light falls on the surface of a refracting medium at critical or polarizing angle, the reflected ray, perpendicular to the refracted ray, is completely polarized.

From Snell’s law we know that

Here, the incident angle is .

If the medium of the incident ray is air then . Let

Now,

This is called Brewster’s Law.