CBSE Class 10 Science 2017 · Paper 31(B)/C · Question 24

(a) Angle of deviation: the angle between the direction of the incident ray and the direction of the emergent ray when light passes through a prism (the angle between the incident ray produced forward and the emergent ray produced backward). White light is a mixture of colours, and light of each colour travels at a different speed in glass, so the refractive index of glass is different for each colour. Each colour is therefore bent through a different angle by the prism, red the least and violet the most, and the colours emerge along different paths and spread into a spectrum (dispersion). The spectrum can be recombined by placing a second, identical prism in an inverted position (upside down) close to the first; it bends each colour back, and white light emerges from it (Newton's experiment). (b) A rainbow is a natural spectrum appearing in the sky after a rain shower. It is caused by the dispersion of sunlight by tiny water droplets in the atmosphere: each drop, acting like a small prism, refracts and disperses the sunlight, reflects it internally and refracts it again as it comes out. Two essential conditions: 1. The Sun must be behind the observer (the rainbow forms in the direction opposite to the Sun). 2. There must be water droplets in the air in front of the observer, lit by the sunlight, as just after a rain shower.