Atmospheric Refraction
The refraction of light caused by the earth’s atmosphere is called atmospheric refraction.
The physical conditions of the refracting medium (air) are not stationary.
Some of the air layers are cold and act like a denser medium whereas other layers, of the atmosphere are comparatively warm and act like a rarer medium.
In the atmosphere the air layers have different optical densities.
So when the light rays pass through the air layers of different optical densities, then refraction of light takes place.
For example:
The air just above the fire becomes hotter than the air farther up.
The hotter air is lighter (rarer medium) than the cooler air (denser medium) above it, and has a refractive index slightly less than that of cooler air.
Since the physical conditions of the air are not stationary, therefore when we see the objects through hot and cold air layers then refraction of light takes place due to which the position of the objects fluctuates.
Some of the optical phenomena in nature which take place due to the atmospheric refraction of light are as follows:
TWINKLING OF STARS
The twinkling of stars is due to the atmospheric refraction of star’s light.
Since stars are very far away heavenly bodies and therefore are considered single point sources of light.
When the light coming from a star enters the earth’s atmosphere, it undergoes refraction due to the varying optical densities of air.
The continuous changing conditions of the atmosphere, refracts the light coming from the stars by different amounts from one moment to another.
When the atmosphere refracts more star light towards us, the stars appear to be bright and when the atmosphere refracts less star light towards us, the star appears to be dim.
This phenomenon goes on thereby giving rise to twinkling of stars.
PLANETS DO NOT TWINKLE
The planets are much closer to the earth and are thus considered as the collection of infinite point sources of energy.
Therefore the dimming effect produced by some of the point sources of light in one part of the planet is nullified by the brighter effect produced by the other point sources of light in the other part of the planet.
As a result, the total variation in the amount of light entering our eye from all the point sources of light will average out to be zero.
Thereby nullifying the twinkling effect. Hence planets do not twinkle.
THE APPARENT POSITION OF THE STARS IS HIGHER THAN THEIR ACTUAL POSITION
The apparent position of the stars is higher than their actual position due to the atmospheric refraction.
The upper layers of the atmosphere act like a rarer medium whereas the lower layers which are close to the earth act like a denser medium.
As the star light enters from rarer to denser medium it bends more towards the normal.
Since the atmosphere bends starlight towards the normal, the apparent position of the star is slightly different from its actual position, as a result, star appears slightly higher than its actual position, as shown in the figure.
ADVANCE SUNRISE AND DELAYED SUNSET
The sun is visible to us about 2 minutes before the actual sunrise and 2 minutes after the actual sunset because of atmospheric refraction.
The actual sunrise takes place when the sun is just above the horizon.
When the sun is slightly below the horizon, the sun’s light coming from rarer medium (i.e. from less dense air) to denser medium (i.e. to more dense air) is refracted downwards as it passes through the atmosphere.
Because of this atmospheric refraction, the sun appears to be raised above the horizon whereas it is actually slightly below the horizon (as shown in the figure).
It is again due to the atmospheric refraction that we can see the sun for about two minutes even after the sun has set below the horizon.
Because of this atmospheric phenomenon, the time from sunrise to sunset is lengthened by about 2 + 2 = 4 minutes.