Atmospheric Refraction

Question 1 What is atmospheric refraction?

Question 2 What causes atmospheric refraction?

Question 3 Why the Sun can be seen about two minutes before actual sunrise?

Question 4 Explain why if we look at objects through the hot air over a fire, the objects appear to be moving slightly?

Question 5 Why do stars twinkle at night in sky?

Question 6 Why planets do not twinkle at night?

Question 7 Name the effects produced by the atmospheric refraction?

Question 8 Explain why stars Seem Higher Than They Actually Are? 

Atmospheric refraction

When light goes from one medium to another medium having different optical densities, then refraction of light rays (or bending of light rays) takes place. All the air in the atmosphere is not at the same temperature. Some of the air layers of the atmosphere are cold whereas other air layers of the atmosphere are comparatively warm (or hotter). 

 

The cooler air layers of the atmosphere behave as optically denser medium for light rays whereas the warmer air layers (or hotter air layers) of the atmosphere behave as optically rarer medium for the light rays. So, in the same atmosphere we have air layers having different optical densities. And when light rays pass through the atmosphere having air layers of different optical densities, then refraction of light takes place. 

The refraction of light caused by the earth’s atmosphere (having air layers of varying optical densities) is called atmospheric refraction.

If we look at objects through the hot air over a fire, the objects appear to be moving slightly. The air just above the fire becomes hotter (than the air further up). This hotter air is optically rarer but the colder air further up is optically denser. So, when we see the objects by the light coming from them through hot and cold air layers having different optical densities, then refraction of light takes place randomly due to which the objects appear to be moving slightly.

Under normal circumstances, the air in the upper atmosphere is optically rarer and as we come down, the air in the lower atmosphere is optically denser.

Some of the optical phenomena in nature which occur due to the atmospheric refraction of light are:

Twinkling of Stars

Stars emit their own light (called star-light). Due to this light, the stars shine in the night sky.When we look at a star in the sky on a clear night, we observe that the intensity of light coming from it changes continuously. At one moment the star appears to be very bright, and the next moment it becomes very dim. And we say that the stars twinkle at night. The twinkling of a star is due to the atmospheric refraction of star’s light.

When the light coming from a star enters the earth’s atmosphere, it undergoes refraction due to the varying optical densities of air at various altitudes. The atmosphere is continuously changing (due to which the optical densities of air at different levels in the atmosphere keep on changing).

 

The continuously changing atmosphere refracts the light from the stars by different amounts from one moment to the next. When the atmosphere refracts more star-light towards us, the star appears to be bright and when the atmosphere refracts less star-light, then the star appears to be dim. The star-light reaching our eyes increases and decreases continuously due to atmospheric refraction.

Stars twinkle at night but planets do not twinkle at all. The stars appear very, very small to us (because they are very, very far off). So, stars can be considered to be point sources of light. The continuously changing atmosphere is able to cause variations in the light coming from a point-sized star (due to refraction) because of which the star appears to be twinkling. 

The planet appears to be big to us (because they are much nearer to the earth). So, a planet can be considered to be a collection of a very large number of point sources of light. 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 point sources of light in its other parts. Thus the whole, the brightness of a planet always remains the same and hence it does not appear to twinkle.

The continuously changing atmosphere is unable to cause variations in the light coming from a big-sized planet (due to refraction) because of which the planet does not twinkle at all.

The Stars Seem Higher Than They Actually Are

Due to atmospheric refraction, the stars seem to be higher in the sky than they actually are. Light from a star is refracted (bent) as it leaves space (a vacuum) and enters the earth’s atmosphere. Air higher up in the sky is rarer but that nearer the earth’s surface is denser. So, as the light from a star comes down, the dense air bends the light more. Due to this refraction of star’s light, the star appears to be at a higher position. Our nearest star, the sun, also seems higher than it actually is, due to atmospheric refraction.

Advance Sunrise and Delayed Sunset

Sometimes the refraction (or bending) of the light rays tend to bring into view the objects which are actually below the horizon (and cannot be seen otherwise).  Thus, we can see the sun about 2 minutes before the actual sunrise and 2 minutes after the actual sunset because of atmospheric refraction.

Let us take the case of sunrise: The actual sunrise takes place when the sun is just above the horizon. But due to refraction of sunlight caused by the atmosphere, we can see the rising sun about 2 minutes before it is actually above the horizon. When the sun is slightly below the horizon, then the sun’s light coming from less dense air 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 when actually it is slightly below the horizon.

It is also due to atmospheric refraction that we can still see the sun for about 2 minutes even after the sun has set below horizon. We can see the sun 2 minutes before the actual sunrise time and for 2 minutes after the actual sunset time. So, the time from sunrise to sunset is lengthened by about 2 + 2 = 4 minutes because of atmospheric refraction.

The sun appears flattened (or oval) at sunrise and sunset. The apparent flattening of the sun’s disc at sunrise and sunset is also due to atmosphere refraction.