^Small angle deviation

δ = A (μ – 1)

δ depends on i, A, μ_prism, μ_medium,

The cause of dispersion is that the speeds of different colours of light in a transparent medium are different.

Also as μ_v > μ_r thus δ_v > δ_r   i.e. the angles of deviation for different colours are different. It is maximum for violet & is minimum for red colour.