optics - Why do convex lenses not disperse light like prisms, given that entry and exit points aren't parallel?

I understand that light entering a parallel block of glass at a non-90 degree angle will cause dispersion of colours within the block but that these will be refracted by the same degree upon exit so there will be no overall dispersion and will appear white. But in that case, why don't concave (or convex) lenses disperse?

Additionally, with a block of parallel glass, if it were sufficiently thick and wide, although different frequencies would eventually "catch up" to each other and merge upon exit, this would occur only after a distance equal to the distance they were dispersed inside the glass right? So the human eye, if positioned close enough to the exit side of the glass, would be able to see a rainbow correct?

Answer

They do. It's called chromatic aberration - each different frequency has a slightly different focus point, blurring the image by different amounts for the different colors. Modern lenses of high quality have multiple elements added specifically to address the issue of chromatic aberration.

What happens with flat glass isn't chromatic aberration - that's an illusion from working with geometric optics. Basically, to get chromatic aberration the rays of light have to exit the glass at different angles, which doesn't happen with glass that is flat on both sides.

It's helpful to think of it in terms of the wave fronts instead of the ray paths, because that is closer to the physical optics. The glass messes up the phase relationship between waves of a different color, but our eyes aren't sensitive to that, anyway. A spherical wave front on one side will be spherical on the other, for all colors (same for flat). All spherical wave fronts that share a center on one side of the glass will also share a center on the opposite side.

What, effect, then, does the displaced ray paths through the glass correspond to? Well, I haven't done the math on this one, so take it with a grain of salt, but I'd bet what it means is that the light that has more spread out ray paths will be dimmed more by the glass than the light that isn't.