Question
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Star twinkle but planet do not twinkle because:
A. star emit their own light but planets receive light from the stars
B. stars do not form a part of solar system
C. stars form a point source of light while planets are considered as a collection of a large number of point source of light
D. during refraction of star light from the atmosphere, star light bends more towards the normal as compared to the planets

Answer
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Hint: In this question first of all we will define refraction of light and then atmospheric refraction. Then we will give the explanation how the twinkling occurs in stars but not in planets.

Refraction of light: - The bending light rays when it passes from one medium to another medium with different optical density is known as refraction of light.

Atmospheric refraction: - The phenomenon of refraction in the atmosphere due variable optical densities of gases present in the atmosphere is called atmospheric refraction.

Twinkling of star: The phenomenon of twinkling of stars is due to the atmospheric refraction of light rays coming from the stars.

The light coming from the stars keeps on refracting from entering the atmosphere of earth until reaching the surface of earth. Atmospheric refraction occurs only when the refractive index of medium is continuously changing. Atmosphere bends the light rays towards the normal that is why the real position of stars is different from the apparent position that appeared to us.
The apparent position of the star keeps on changing because of the variation in optical density of the atmosphere.
Since the stars are very far away from the earth surface. They are considered to be a point source and the position of these point sources keep on deflecting which appear to us as a twinkling of stars. Sometimes stars appear brighter and sometimes they appear dim.

Why does the planet not twinkle?
Planets do not twinkle as the star does. This is because the planets are not so far away as the stars are. They are not considered to be point sources but they are considered as the collection of point sources of which the average deflection of light coming from them is zero.

Hence, option \[(C)\]is correct.

Note: - It is important to note that before answering this question we should be aware of refraction, atmospheric refraction.