# Dispersion of light through a prism but not through glass or lense

Why does dispersion happen in prisms but not in glasses nor lenses?

Dispersion happens in glasses and lenses, so no explanation is needed. For example optical telescope lenses are made of several elements which are designed to counteract the effect of dispersion, also known as chromatic aberration.

The wording of your question shows a confusion between dispersion and refraction.

Dispersion only depends on the glass material, not its shape. However the triangular prism has a shape maximising the effect of refraction: each wavelength in an incoming white light entering the prism is first refracted when entering the prism, thus travelling at a specific angle within the glass, then refracted again when exiting the prism. The overall effect is a nice rainbow-like angular dispersion of all wavelengths of the incoming light.

If you do not have refraction at all (flat glass hit at right angle) or only little of it (lenses) the effect of dispersion is much less visible because there is less (or no) eventual angular separation of the different wavelengths.

For a detailed discussion of prism physics, see physics.stackexchange.com/q/65812.

You can think of a simple thin-lens, to first-order, as 2 prisms joined together. So, of course there would be dispersion (ie. change in index of refraction versus wavelength) and hence the focal distance will change.

An important aspect of this discussion is that different materials will have different dispersions. So, perhaps when a glass or lense material is chosen it is done so in such a way as to minimize the dispersion observed by the human.

Primes, glasses, lenses are made of the same material, and are all dispersive medium.In a dispersive media, the phase velocity of a wave depends on its frequency. So different frequencies(colors) of wave will travel in different velocities. we can clearly see the effect in the prism because its dispersive relation is greatest.

http://www.daviddarling.info/encyclopedia/A/achromatic.html Chromatic aberration can be mitigated by layering two types of glass, namely: Flint glass and Crown glass. Whether they go to this trouble with spectacles can be answered by lens makers. Probably depends upon the price

• This doesn't really seem to answer the question – Kyle Kanos May 25 at 1:42