We know from the concept of dispersion of light, When a beam of white light incident on a prism, it disperses all the colours at different angles, So the emerging beam will be having a coloured band, in which all the colours are arranged according to their wavelengths. If one more prism is arranged in opposite manner, then the dispersed colours will merge into white light beam again. This is shown in the figure below. Here the extreme colours only shown. All the colours will be arranged as Violet, Indigo, Blue, Green, Yellow, Orange and Red from down to up. When these passes from the second prism, all join together and gives a white light as in the figure.
