Of his rainbow experiment Newton wrote that he had projected white light through a prism onto a wall and had a friend mark the boundaries between the colors, which Newton then named. In his diagrams, ...

This means that the light leaving the prism is spread out into its different colours, a process called dispersion. This experiment is recorded as first being carried out by Isaac Newton ...

The question was why. The common explanation for the appearance of the colours was that they were impurities added by the prism to the pure white light. Newton thought that the colours were ...