CBSE Class 10 Science 2019 · Paper 31/1/1/C · Question 20(b)

(a) Newton passed a narrow beam of sunlight through a glass prism and obtained a band of seven colours (VIBGYOR) on a screen: the prism disperses white light. When he passed one colour of this spectrum through a second similar prism, no new colours appeared. He then placed a second prism, identical to the first, in an inverted position next to it (see the diagram). All the colours passed through the second prism, which bent them back, and a beam of white light emerged from its other side. So the seven colours together make white light: sunlight is made up of seven colours. (b) Atmospheric refraction is the refraction of light by the Earth's atmosphere, whose layers of air have gradually changing optical density (refractive index). Two natural phenomena based on it: (i) twinkling of stars; (ii) advance sunrise and delayed sunset - the Sun is seen about $2$ minutes before it actually rises and about $2$ minutes after it actually sets. (Stars appearing higher than they are, and the flattened look of the Sun at sunrise and sunset, are also accepted.)