The phenomenon of splitting of white light into its constituent colours on passing through a prism is known as the dispersion of white light. This splitting of the light rays occurs because of the different angles of bending for each colour and this differentangles of bending occurs because different component of light faces different refractive index (or speed of different colours is different) when passing through the glass prism.It is essential that the two prisms used for the purpose should be identical and placed in an inverted position with respect to each other so that the second prism completely nullifies thedispersion caused by the first prism and we get pure white light.