As the refractive index of a transmittive medium is dependent on the wavelenght of light, dispersion occurs, when white light transmits such a medium. Refraction is stronger for light of short wavelengths, for example blue, and less intensive for light of long wavelngths, for example red.
Different kinds of glasses cause refraction or dispersion of various intensities. In photographical lenses the the same effect occurs. So it is neccessary to correct these abberations, otherwise you get to see these effects in the images.