The phenomena of splitting of light into its component colours is known as dispersion and its extent of dispersion is called dispersive power.
In the experiment Newton put another similar inverted prism and let the emergent beam from the first prism fall on second prism. The resulting colours into component lights and the second inverted prism, recombines it to form a single white light. Dispersion takes place because the refractive index of medium for different colour is different.
