CBSE Class 10 Science 2017 · Paper 31/3/C · Question 11

Main cause of atmospheric refraction: the air is made up of layers of different temperature and density, and hence of different refractive index (hot air is less dense and has a slightly lower refractive index than cooler air; the air is generally denser near the Earth), and these conditions keep changing. Light passing through the atmosphere is therefore refracted continuously. Twinkling of stars: starlight entering the atmosphere passes into denser and denser layers and bends towards the normal each time, so it reaches the eye along a steeper path, and the star is seen at an apparent position slightly higher than its actual position (see the diagram). Since the physical conditions of the atmosphere keep changing, the path of the starlight keeps changing slightly: the apparent position of the star fluctuates, and the amount of starlight entering the eye flickers — the star appears brighter at one moment and fainter at the next. Stars are so far away that they act as point sources, so this flickering is seen as twinkling.