Newton’s Experiment on Dispersion: Sir Isaac Newton gave an explanation for dispersion. He placed a similar prism in an inverted position. The emergent beam from the first prism was made to fall on the second prism (Figure). The resulting emergent beam was found to be white light.
Fig: Newton’s experiment on dispersion
The first prism separated the white light into its constituent colours, which were then recombined by the inverted prism to give white light. Thus it can be concluded that the prism does not create any colour but it only separates the white light into its constituent colours.
Dispersion takes place because the refractive index of the material of the prism is different for different colours (wavelengths). The deviation and hence the refractive index is more for violet rays of light than the corresponding values for red rays of light. Therefore the violet ray travels with a smaller velocity in glass prism than red ray. The deviation and the refractive index of the yellow ray are taken as the mean values.