CBSE Class 10 Science 2017 · Paper 31/1/C · Question 20

Spectrum: when a beam of white light passes through a glass prism it splits into its component colours; this is dispersion. The band of coloured components so obtained is called the spectrum of white light. Its colours, in order, are violet, indigo, blue, green, yellow, orange and red (VIBGYOR). Different deviations: light of different colours travels with different speeds in glass, so the refractive index of glass is slightly different for each colour, least for red and greatest for violet. Each colour is therefore bent through a different angle at the faces of the prism: red is deviated the least and violet the most, so the colours emerge along different directions and spread out. Recombination: a second prism, identical to the first, is placed in an inverted position (upside down) close to it, so that all the colours coming out of the first prism pass through it. The second prism deviates each colour by the same amount in the opposite direction, so all the colours emerge together, parallel to the original beam, and recombine to give a beam of white light. This is Newton's experiment.