Question

Explain the process of twinkling of stars?

Answer 1

Stars twinkle due to atmospheric refraction of starlight through the earth’s atmosphere. Due to refraction, the apparent position of a star is different from its actual position. The apparent position of the star changes due to refraction in the atmosphere, so the star sometimes appears brighter and sometimes fainter.

Complete answer:
Refraction is the bending of light when it passes from one transparent medium to another. Earth’s atmosphere has a different concentration in different places. So the refractive index of Earth’s atmosphere gradually changes. When starlight enters into the earth’s atmosphere it undergoes repeated refraction. Since during atmospheric refraction, the starlight bends towards normal, so the apparent position of the star is slightly different than the actual position. But the position of stars is not stationary but keeps on changing. Also, stars are very distant away from Earth. So they can be approximated as point size light-sources. As the position of stars changes the path of starlight changes, due to which the amount of light entering our eye flickers (i.e. sometimes it appears brighter and sometimes fainter) which produces a twinkling effect.

Additional Information: When light travels from one transparent medium to another it bends at the junction point. The perpendicular drawn at this point Is called normal at the point of refraction. If a light ray travels from a medium of lower refractive index to a medium of higher refractive index, it bends towards the normal and if a light ray travels from a rarer medium to a denser medium at the point of refraction it bends away from normal.

Note: Note that the twinkling effect only occurs in stars not in planets, because the planets lie much closer to Earth in comparison with stars. So planets are seen as the extended (or large) sources. If you consider a planet as a collection of point sized sources, the total amount of variation from every point will be average-out to zero. So the twinkling effect does not apply to planets close to the earth.