Question

Write short notes on:

(1) Refraction observed in the atmosphere.

(2) Dispersion of light.

Answer 1

(1) As we know when light travels from a rarer to a denser medium or vice versa, it gets deviated from its straight line path. A necessary condition here to be noted is that the mediums should be of varying densities.

Now consider the atmosphere. The density of air is not uniform throughout the atmosphere. Hence, whenever light from the Sun passes through the atmosphere, it encounters different packets of air of varying density along its incoming path and hence bends itself.

This phenomenon is responsible for twinkling of stars and the apparent red colour of the Sun during sunrise and sunset. See figure below:

The yellow star is the actual position of the star. It is not perpendicular to the horizon, due to the bending of light. The observer sees the star’s image above the horizon even before it rises.

(2) Dispersion: The phenomenon of the splitting up of white light into its constituent colours is known as dispersion.

For example, when light passes through one face of a prism it gets deviated.

This deviation is different for different wavelengths of light. Since different colours of light have different wavelength, the deviation for different colours is different. As a result, when the light emerges from the other face of the prism, we observe the seven constituents of light. This phenomenon of splitting of light into its constituent colours is called dispersion.

It is not necessary that only white light disperses. Any light that contains more than one component of colour can get dispersed.