Monochromatic RGB light sources passing through a prism

[KaylaT]

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?

 

[mfb]

For a prism, this is usually not the case. For a block where entry and exit surfaces are parallel to each other, it is true, and follows from symmetry.

 

[Cutter Ketch]

For a prism, this is usually not the case. For a block where entry and exit surfaces are parallel to each other, it is true, and follows from symmetry.

To expand on what mfb said, anything that disperses white light into a spectrum will also spread your colors into a fan. Your premise is flawed. Mfb was trying to guess where you got this wrong idea. He thinks perhaps you were confused by an illustration where the entrance and exit faces are parallel. Here neither white light nor the three colors are fanned out. The rays emerge parallel as you describe.