Select the correct statements :

            I) There is no dispersion of light refracted through a rectangular glass slab

            II) Dispersion of light is the phenomenon of splitting of a beam of white light into its

                 constituent colours

            III) Dispersion of light occurs, because velocity of light in a material depends upon its colour.

            IV)The dispersive power depends only upon the material of the prism, but not upon the

                  refracting angle of the prism