We know that 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. Now, in the atmosphere, we have air everywhere. But 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). Now, the coolar 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.
We can see the effect of atmospheric refraction by simple observation as follows:It we look at objects through the hot air over a fire, the objects appear to be moving slightly. This can be explained as follows:
The air just above the fire becames 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 to which the objects appear to be moving slightly. This is an example of atmospheric ref refraction on a small scale.
Please note that under normal circumstances, the air upper atmosphere is optically rarer and as we come down, the air in the lower atmosphere is optically denser. This arrangement of optically densities of air in the atmosphere can, however, change according to the local conditions such as temperature, etc., at a particular place. We will now describe some of the optical phenomena in which occur due to the atmospheric refraction of light.
* Twinkling of Stars
We know that stars emit their own light (called star - light). Due to this light, the stars shine in the night sky. Now, 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 becames very dim. In fact, the stars go on becoming bright and dim, bright and dim, again and again. And we say that the stars twinkle at night. The twinkling of a star is due to the atmospheric refraction of star's light. This can be explained as follows:
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 amount from one moment to twinkle eyes increases and decrease continuously due to atmospheric refraction. And the star appears to twinkle at night.
We know that through stars twinkle at night but planets do not twinkle at all. This can be explained as follows: 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 variation in the light coming from a point - sized star (due to refraction) because of which the star appears to be in the light coming from a point -sized star (due to refraction) because of which the star appears to be twinkling. On the other hand, the planets appear to be quiet big to us (because they are much nearer to the earth). So, a planets 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 planets is nullified by the brighter effect produced by the point sources of light in its other part. Thus, on the whole, the brightness of a planet always remains the same and hence it does not appear to twinkle. We can now say that: The continuously changing atmosphere is unable of which the plants does not twinkle at all.
