Atmospheric refraction

By - Rishita Agarwal

Pooja Jalan

Subhasree Mukherjee

Atmospheric refraction is the deviation of light or other electromagnetic wave from a straight line as it passes through the atmosphere due to the variation in air density as a function of altitude.

This refraction is due to the velocity of light through air decreasing (the index of refraction increases) with increased density.

Atmospheric refraction near the ground produce mirages and can make distant objects appear to shimmer or ripple, elevated or lowered , stretched or shortened with no mirage involved

Shimmering of face

• When we sit around a campfire , the face of the person opposite to us appears to shimmer.

• This happens due to refraction of light. The rays of light reflected from the face of the person sitting opposite to us , on passing through hot air , gets refracted. Since hot air is rapidly moving and its optical density is continuously changing . Therefore the path of refracted rays also changes.

• This gives shimmering effect.

Advanced sunrise and delayed sunset

• The sun is visible to us before the actual sunrise and after the sunset.

• The density of air near the surface of the earth is slightly higher. The rays of the sun coming through space enter at the top of the earth’s atmosphere and travel from rarer to denser medium.

• They bend slightly towards normal at each refraction and appear to come from apparent position of sun. Hence , sun appears to rise a few minutes before the actual rise . The same phenomenon applicable for delayed sunset as the sun continues to be seen a few minutes after it has actually set.

Apparent position and twinkling of stars

• The starlight on entering earth’s atmosphere undergoes refraction continuously before it reaches the observer.

• Since the refractive index of the earth’s atmosphere is gradually changing , the atmosphere bends the starlight towards the normal, and the apparent position of the star is slightly higher than its actual position.

• As the path of rays of the light coming from the stars goes on varying slightly , the apparent position of the star fluctuates and the amount of light entering the eye flickers.

Planets do not twinkle

• Planets are very close to us as compared to stars. This apparent

position also changes with the change in the density of

different layers of atmosphere. However , the size of their

apparent image is still fairly large , such that it seldom falls

outside the line of sight. Hence , they do not appear to twinkle.

Sun appears bigger during sunset or sunrise

• If the sun (or moon ) is low above the horizon , the optical path of the light through the atmosphere is very long thus refraction is also maximum.

• Hence , apparent image of the sun is very much closer to the eye . Thus, it appears bigger.

Sun appears flattened at sunrise or sunset but circular at noon

• At sunset and sunrise , the sun is near the horizon. The rays of light from the upper and lower edge of the sun bend unequally while travelling through the atmosphere. As a result of this phenomenon , the sun appears oval or flattened.

• At noon , the sun is overhead. The rays of light from the sun enter the atmosphere normally and do not bend. So the sun appears circular.

Mirage

• Warm air is less dense than cool air and the difference between them creates a gradient in the refractive index of air.

• Light from the sky at a shallow angle to the road is refracted making it appear as if the sky is reflected by the road’s surface.

• The mind interprets this as a pool of water on the road as water also reflects the sky.