A medium is said to be dispersive if ____
The splitting of white light into its component colours when passing through a refractive medium is Dispersion'. The cause of this phenomenon is the change in the speed of light with wavelength. ie., in a refracting medium, lights of different colour travel at different speed. The speed of violet colour(least wavelength) is the least and of the red colour(greatest wavelength) is the greatest. For example, when light travels from air into water, it slows down, causing it to continue to travel at a different angle or direction and so they split into the colour spectrum.