When a collimated beam of white light enters a rectangular prism, it develops slightly coloured edges due to the dispersive refraction of the glass: each ray in the beam is displaced differently according to its colour, and while in the middle of the beam this is compensated by the neighbouring rays, at the edges of the beam this is not compensated and the beam develops a red and a violet edge.

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In answering this question I looked for photos of this effect and didn't find any. Now I wager this site has a large population of people with ready access to labs where taking nice pictures of this effect is pretty trivial. Can I tempt any of you to enrich the internet a bit and take such a picture?


1 Answer 1


Interesting question because the diagram is correct but I too have never seen this in the lab.

It turns out that the dispersion of the material and its thickness has to be very large to observe this.

Here is a ray trace simulation:

  • Bar geometry, 7.5cm x 1cm,
  • Glass SF11 (Highly dispersive with Abbe number ~25),
  • Ray wavelengths 380nm (blue) and 780nm (red).

Interestingly, the rays come to a focal point after exiting. I have added a few mirrors so this can be observed in the same image as the slab.

Ray Trace simulation of refraction slab.


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