It is well known that chromatic dispersion occurs when a white light source is incident on a prism. The index of refraction (of the prism) is greater for shorter wavelengths hence the colours emerge from the other side with an angular spread. However I am interested to know the detailed reason why when monochromatic light of different wavelengths (say red, green and blue) is incident at the same time on the prism, the emergent rays are parallel to each other, that is there is no dispersion seen as with the white light source. Why would the refracted angles of these three rays be the same even though they have different wavelengths? I would've thought that they would still emerge at different angles with respect to each other, similar to that with the prism but without the other colours involved. Where's my gap in knowledge?