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, ...

but what set Newton apart was the fact that he devised and performed an experiment to test his hypothesis. He shone a white source of light through a prism and as expected obtained a rainbow.

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 ...