What is Atmospheric Refraction ?
We know that when light travels from one medium to another, refraction takes place. In the atmosphere, the layers of air have different optical densities. That means that different layers of air have different refractive index. Air in upper atmosphere is optically rarer but air in lower atmosphere is optically denser. So, when light rays pass through air, refraction takes place.
Refraction of Light due to different layers of earth's atmosphere is called Atmospheric Refraction.
Twinkling of the stars and Stars appear higher than they actually are :
Twinkling of the stars is basically due to the process of atmospheric refraction. When the light from a star travels through space and reaches the earth’s atmosphere, it undergoes refraction and bends a little.
This speed of the light decreases significantly as the light enters from a vacuum into the atmosphere. The human brain produces the image of the star by shifting it in the forward direction than the original position of the star. Hence, the atmospheric reflection of light leads to a phenomenon called the apparent shifting of the star.
The light originating from the star does not follow a straight path to reach earth but tends to take a zig-zag path. This happens because of the uneven density of the earth’s atmosphere. The factors responsible for the uneven density are the temperature and composition of the air. The greater be the temperature, the lesser will be the density and vice-versa. Similarly, the air contains a disproportionate amount of gases and dust particles, which affect the density and hence, the speed with which the light from the star travels.
Hence, the change in density of the earth’s atmosphere corresponds to a change of path taken by the light from the star, which further leads to the twinkling of the stars. The atmospheric refraction also affects the brightness of a star. When the atmosphere refracts more starlight towards the observer, it appears bright; whereas, when it refracts less light, the star appears dull.
Explanation for Advanced Sunrise and Delayed sunset :
Before Sunrise, sun is below the horizon. Sun's rays are coming from more dense air towards less dense air. This causes sun rays to refract and appear to come above the horizon. Our eyes see sun rays as a straight line, it appears to us that sun has risen but it has actually not yet risen.
After Sunset, Sun is below the horizon Sun's rays are coming from more dense air towards less dense air This causes sun rays to refract and appear to come above the horizon. Our eyes see sun rays as a straight line, it appears to us that sun has not yet set.
Thus, the Sun appears to be raised and can be seen 2 min before actual sunrise and 2 min after actual sunset.