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Dispersion by Prism

A beam of white light, when it passes through a prism is split up into its constituent colours and this is called dispersion of light. The image thus formed on a screen is called a spectrum.

The spectrum consists of visible and invisible regions. In the visible region the order of the colours is from violet to red. The principal colours are given by the word VIBGYOUR (Violet, Indigo, Blue, Green, Yellow, Orange and Red). The deviation produced for the violet rays is maximum and for red rays of light it is minimum. Fig. represents the dispersion of a white ray of light by a prism in the visible region. The region of the spectrum, of wavelengths than violet is called ultra-violet and the region of wavelengths longer than red is called infra-red. In present, the discussion relates only to the visble region of the spectrum.

The refractive index for the material of a prism (or a lens) is different for different wavelengths (or colours). The deviation and hence the refractive index is more for blue rays of light than the corresponding values for red rays of light. The deviation and the refractive index of the yellow constituent are taken as the mean values. If the dispersion through a prism does not follow the order given by VIBGYOR, it is said to be anomalous dispersion.