This is because different colours have different wavelength and therefore scater with different dispersion rate . I hope u understood .sry lot of technical words
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Mono chromatic light is used for a glass prism because monochromatic light doesn't split into its component colours as a result of which the process of refraction can be easily observed in contrast if a polychromatic light is used then due to refraction there will be splitting of such a light into its component colours