CBSE Class 10 Science 2017 · Paper 31/1/2/C · Question 17

Refraction in the atmosphere: The atmosphere is not uniform. Its density, and so its refractive index, is greatest near the ground and decreases with height, and it also changes with temperature (hot air has a lower refractive index than cooler air). Light passing through it therefore travels through a medium of gradually changing refractive index and is refracted continuously; starlight coming down into denser air bends towards the normal and follows a curved path. Because the air keeps moving, the amount of bending keeps changing. Stars twinkle: Because of this refraction, a star is seen slightly away from its actual position. As the atmosphere keeps changing, the path of the starlight varies slightly, so the apparent position of the star fluctuates and the amount of light entering the eye flickers: the star appears brighter at one moment and fainter at the next. Stars are so distant that they act as point sources, so this flicker is seen as twinkling. Planets do not twinkle: Planets are much closer to the Earth and are seen as extended sources, each a collection of a large number of point sources. The variations in the light from the individual points average out, so the total light entering the eye stays nearly steady and there is no twinkling.