CBSE Class 10 Science 2017 · Paper 31/2 · Question 17

Dispersion of white light is the splitting of white light into its seven component colours — violet, indigo, blue, green, yellow, orange and red — when it passes through a transparent medium such as a glass prism. The band of colours obtained is called the spectrum. Cause: different colours of light bend through different angles with respect to the incident ray as they pass through the prism, because the refractive index of glass is different for different colours (greatest for violet, least for red). Red light bends the least and violet the most, so the colours emerge along different paths and separate. Ray diagram: a narrow beam of white light falls obliquely on one face of the prism; it bends towards the base at both faces and emerges as a spread of colours, with red deviated the least (at the top of the spectrum) and violet deviated the most (at the bottom), as in the figure.