Question

(i). Describe briefly, with the help of a suitable diagram, how the transverse nature of light can be demonstrated by the phenomenon of polarization of light.
(ii). When unpolarised light passes from air to a transparent medium. Under what conditions does the reflected light get polarized?

Answer 1

Light is made up of electric and magnetic waves that undergo oscillations perpendicular to each other, in polarized light, the oscillations are confined to a single plane. The intensity of polarized light is less than unpolarized light. Different mediums can be used to polarize light.

Complete solution:
Light is an electromagnetic wave; it has electric and magnetic fields oscillating perpendicular to each other. That is why the waves are oscillating in two planes.

Light is also a transverse wave, this means it propagates by forming crests and troughs. This can be depicted through the following experiment.

Light rays in which the oscillations are restricted to a single plane is called polarized light.

Take a tourmaline crystal and light from a source is made to fall on the flat surface of the crystal. The light falls normally on the surface, parallel to the axis. We see that only a part of light is transmitted through the crystal, this means that the intensity of light decreases.

Now another similar crystal is taken and kept at a distance from the first one. The light is made to transmit through both planes and the light coming out of the last crystal remains unchanged. Now the latter crystal is rotated slowly in this case the light coming out decreases in intensity until it completely vanishes when the second crystal is perpendicular to the first crystal. Similarly, the intensity again starts to increase when we further rotate it.

Therefore, the above experiment shows us that the waves are transverse because if the waves were longitudinal, the intensity of light coming out of the last crystal would remain unchanged.

(ii). When light travels from air to a transparent medium, the path of light changes due to which it undergoes refraction but some light rays also get reflected. The reflected light rays become polarized when the reflected as well as the refracted rays are perpendicular to each other.

Therefore, for polarization to occur the reflected and refracted rays must be perpendicular.

Note:
When the reflected and the refracted rays are parallel to each other, the tangent of the angle of incidence is equal to the refractive index of the medium; this is called the Brewster’s law. The tourmaline crystal is used to restrict the oscillations in one plane and hence polarize light.