Whn white light passes through a prisim it nor only diviates from its original direction of propagation but also splits up into 7 constitutent colours....this happens because whn white lite travels in air or vacuum all the 7 constitutent colours travel at same speed . The colours remain together and therfore the light appears white. But whn it is incident on a prism all d different colours travel at different speeds and are refracted or deviated by different angles. D diveation is max for violet colour lite and minimum for red colour light..