We cover atmospheric refraction and the phenomenon which happen because of it.
(i) Atmospheric refraction
(ii) Twinkling of Stars
(iii) Advanced Sunrise and Delayed Sunset
1. Atmospheric refraction is the deviation of light from a straight line as it passes through the atmosphere due to the variation in air density which changes with height.
(i) Cold layers of the atmosphere have higher density and therefore higher Refractive Index. Whereas, hotter layers are lighter and therefore less dense and have lower Refractive Index. The atmosphere consists of many hot and cold layers one above the other.
(ii) Due variation in densities of different layers of atmosphere and as the physical conditions of the refracting medium (air) is not stationary, the apparent position of the object, as seen through the hot air, fluctuates.
(iii) This wavering is thus an effect of atmospheric refraction (refraction of light by the earth’s atmosphere) on a small scale in our local environment. The twinkling of stars is a similar phenomenon on a much larger scale.
Twinkling of stars
1.(i) The twinkling of a star is due to atmospheric refraction of starlight.
(ii) The starlight, on entering the earth’s atmosphere, undergoes refraction continuously before it reaches the earth.
(iii) The atmospheric refraction occurs in a medium of gradually changing refractive index.
(iv) Further, this apparent position of the star is not stationary but keeps on changing slightly, since the physical conditions of the earth’s atmosphere are not stationary.
(v) Since the stars are very distant, they approximate point-sized sources of light. As the path of rays of light coming from the star varies slightly, the apparent position of the star fluctuates and the amount of starlight entering the eye flickers. Star sometimes appears brighter, and at some other time, fainter, which is the twinkling effect.
2. Since the atmosphere bends starlight towards the normal, the apparent position of the star is slightly different from its actual position. The star appears slightly higher (above) than its actual position when viewed near the horizon.
3. Planets do not twinkle. The planets are much closer to the earth and are thus seen as extended sources (consisting of many point sized sources). Therefore, the total variation in the amount of light entering our eye from all the individual point-sized sources will average out to zero, thereby nullifying the twinkling effect.
Advance sunrise and delayed sunset
1. Sunrise is defined as the moment that the Sun first appears over the horizon. So, by definition, you can’t see the Sun before it appears.
2. But, the Sun is visible to us about 2 minutes before the actual sunrise, and about 2 minutes after the actual sunset.
3. This is because of the atmospheric refraction of the light from the Sun by the Earth’s atmosphere. Earth’s atmosphere bends the path of the light so that we see the Sun in a position slightly different from where it really is. The magnitude of this effect varies with latitude, but it’s strongest at the equator, where the Sun rises 2 minutes earlier than it would if the Earth had no atmosphere, and sets 2 minutes after it would if the Earth had no atmosphere.
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