(a) What is atmospheric refraction? What causes atmospheric refraction?(b) Why do stars twinkle on a clear night?(c) Explain why, the planets do not twinkle at night.

(a) The refraction of light due to the earth's atmosphere is called atmospheric refraction. Atmospheric refraction is caused by the bending of light rays when they pass through the layers of the earth's atmosphere having different optical densities.

(b) Stars twinkle on a clear night due to atmospheric refraction.

When the light coming from the star enters the earth's atmosphere, it gets refracted (bent), due to varying optical densities of air at various altitudes. The atmosphere keeps changing continuously due to which light from the stars gets refracted by different amounts from one moment to the next.

When the atmosphere refracts more star-light towards us, the star appears to be bright, whereas when it refracts less star-light, then it appears to be dim. In this way, the star-light reaching our eyes increases and decreases continuously due to refraction, and thus star appears to twinkle at night.

Image is posted for reference only

(c) The planets appears to be big to us as they are much nearer to the earth. So, the continuously changing atmosphere cannot cause much deviations in the light coming from the planets. Thus, the brightness of the planets remains the same, due to which they do not appear to twinkle at night.