How does refraction take place in the atmosphere? Why do stars twinkle but not the planets?

Light gets refracted at different layers of atmosphere. The air just above the radiator becomes hotter than the air farther up. Since the hotter air is less dense than the cooler air above it and has less refractive index than cooler air. The physical conditions of the atmosphere are not same, the apparent position of the object through hot air fluctuates. This is known as atmospheric refraction.

Twinkling of stars is due to atmospheric refraction of star light. The star light after entering the earth’s atmosphere undergoes refraction in a continuous manner before it reaches the Earth. The atmospheric refraction is due to change in the refractive index at different level in atmosphere. The star light bends towards the normal, the apparent position is different from the actual position of star. Since atmosphere is not stationary and keeps changing. As path of rays of light coming from star varies slightly then the apparent position of star also varies slightly and the amount of light entering the eye flickers. Sometimes, it is brighter and sometimes the star seems fainter. In this way, stars twinkle but planets don’t twinkle because planets are much closer to Earth and are seen as extended sources. If planet is considered as collection of point sources of light, the total amount of light entering the eye on average is zero and therefore the twinkling effect of planets is nullified.