CBSE Class 10 Science 2020 · Paper 31/2/3 · Question 23(b)

(a) A rainbow is formed by tiny water droplets suspended in the air after a rain shower, which act like small prisms. Sunlight entering a droplet is refracted and dispersed into its colours, is reflected internally at the back of the droplet, and is refracted again as it comes out. Because of the dispersion and the internal reflection, different colours reach the observer’s eye, and the rainbow is seen in the direction opposite the sun. (b) Stars twinkle because of atmospheric refraction. Starlight is refracted continuously as it passes through layers of air whose refractive index keeps changing with the changing physical conditions of the atmosphere. A star is so far away that it acts as a point source, so its apparent position keeps shifting slightly and the amount of its light entering the eye flickers, and the star appears to twinkle. (c) The sky looks blue from the earth because the particles of the atmosphere scatter sunlight, blue light most of all. An astronaut is above the atmosphere, where there is no air (no medium) to scatter sunlight, so no scattered light reaches the eyes from the sky and it appears dark.