CBSE Class 10 Science 2017 · Paper 31/3 · Question 17

Dispersion of white light: the splitting of white light into its seven component colours (violet, indigo, blue, green, yellow, orange, red) when it passes through a prism. It happens because different colours bend through different angles: red the least, violet the most. Recombination: a second prism, identical to the first, is placed in an inverted position next to it. The first prism disperses the white light into a spectrum; the colours pass through the second prism, and a beam of white light emerges from it (labelled diagram). Why identical and inverted: the second prism must bend each colour through exactly the same angle as the first, but in the opposite direction, so that the dispersion is undone and all the colours come out together, parallel to the original beam, as white light. An identical prism gives the same deviation for every colour, and inverting it reverses that deviation; together the two prisms act like a parallel-sided glass slab.