CBSE Class 10 Science 2019 · Paper 31/4/1 · Question 7(a)

Cause: White light is made up of seven colours, which travel at different speeds in glass, so the refractive index of glass is different for each colour. Each colour is therefore bent through a different angle on refraction, violet the most and red the least, and because the two refracting faces of a prism are inclined to each other, the colours spread out into a spectrum. This splitting is dispersion. Ray diagram: The first prism splits the narrow beam of white light into a spectrum (red deviated least, violet most). The second, identical prism, placed inverted, bends each colour back through the same angle in the opposite direction, so all the colours emerge parallel to the incident beam and recombine, and white light emerges from the second prism. Label the incident white light, the two prisms, the spectrum (R to V) between them and the emergent white light, with arrows on the rays.