White light is a combination of 7 colours. Different colours light has different speed and different wavelength. Hence for each wavelength or colours of the light refractive index is different. So they bend at different angles when passes through a prism. Thus white light gets splits into its 7 constituent colours. This process is called dispersion.
Newton allowed passing a thin beam of white light on a prism which gets split into its constituent 7 colours. Then he allowed passing these 7 colours of light through an inverted prism as shown in below figure. Only a beam of white light comes out from the second prism thus he concluded that white light is made up of 7 colours.
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