Question

How does a prism divides white light into seven colours of rainbow that can be seen on a screen?

Answer 1

White colour consists of many colours. Each colour has different wavelength. Prism refracts different wavelength hs at different angles. hence all colurs are seen separately

Answer 2

Due to dispersion of white light the prism splits the white light into its constituent seven colours. The cause of splitting of white light is because, light of different colours have different speeds in glass.