well zulkarnain..
i guess i have answer such questions in detail as as asked by alistu in some post.
let me explain that the wavelength dependence of refractive index of medium.
i.e same medium will exibit the different refractive index for diffferent wavelengths, hence different velocities(which we call as the dispersion)
there is an equation called CAUCHY EQUATION FOR REFRACTIVE INDEX.which reveals that a medium havs different refractive index for different medium, hence different velocities
you can see that
with regards