Why don't dispersion occur in glass blocks?

Latrommi

This question bothered me for some time.

From what i have seen in textbooks, dispersion occurs as soon as white light passes from air into a glass prism, but why not for a glass block, assuming that the angle of incidence is the same?

The standard answer for why dispersion happens in glass prism is because of the different speeds of different coloured light in glass, so why is there a difference between a glass block and a glass prism?

krab

There is no difference. Dispersion does not depend upon shape of the glass.

Latrommi

Silly me.

Yes, it does happen in a glass block, except the phenonmenon is very difficult to observe. A more detailed explanation is given here:

http://www.physlink.com/Education/AskExperts/ae184.cfm

Thks for replying anyway.

Gonzolo

The back side, which is parallel to the front side, reverses the effect.

Doc Al

Not exactly. When light passes through a parallel slab the outgoing beam is parallel to the incoming beam, but its exit point is displaced a bit from its entrance point. Due to dispersion within the slab, each frequency is displaced slightly differently.

Of course a prism gives each frequency a different exit angle, so it's much easier to see the dispersion. (Check out the link that Latrommi posted; it's not bad.)

Gonzolo

I totally agree, but this displacement also happens with non-parallel faces. It is the effect of direction change that is critical here, and that gets reversed with a slab.