CBSE Class 10 Science 2017 · Paper 531/1 · Question 24

Dispersion: when a narrow beam of white light passes through a glass prism, it splits into its seven component colours, violet, indigo, blue, green, yellow, orange and red (VIBGYOR). The glass has a slightly different refractive index for each colour, so the colours are bent through different angles with respect to the incident ray: red is deviated the least and violet the most. They therefore separate and form a band of colours, the spectrum, with violet nearest the base of the prism (see the first ray diagram). Recombination: a second glass prism, identical to the first, is placed in an inverted position beside it so that the spectrum falls on it. It deviates each colour back by the same amount in the opposite direction, so all the colours recombine and white light emerges from the second prism, parallel to the original beam (see the labelled diagram). This showed Newton that white light is made up of seven colours.