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Formation of spectrum of colors when light passes through a flat pane of glass

  1. Here's a question from my textbook: "Why do you not see a spectrum of colors when light passes through a flat pane of glass?"

    However I think that a spectrum of colors will be formed when light passes through a flat pane of glass. The colors will all be parallel to one another, unlike a prism where they are diverging. I think so because at the first face of the flat pane they will all disperse, as happens at the first face of a prism and at the second face they will all be refracted in such a way that they are all parallel to the incident angle and hence parallel to each other. Am I right or do they not disperse?

    PS: The answer in the textbook is, "Because a flat pane of glass has parallel sides"
  2. jcsd
  3. mfb

    Staff: Mentor

    Your explanation is an expanded version of "Because a flat pane of glass has parallel sides". You do not see shifts of light, unless you have some beam with an extremely small focus and a very narrow angular profile. A different angle for different colors is easier to see.
  4. No, my explanation is the opposite of what they said. I'm trying to say that what they said is wrong. They will disperse but the beam will be parallel, not diverging, is what I'm trying to say. That's because they each undergo different lateral shift.
  5. Well....?
  6. Drakkith

    Staff: Mentor

    You are correct in that the colors will be shifted. However I think the textbook is simply pointing out that in a prism under the same conditions you would see a spectrum of colors, whereas in a flat piece of glass you will not be able to notice the color shift since it is very very small.
  7. Another explanation given in another book says that if you cur it along the diagonal you can imagine it to be two prism and you already know how recombination occurs with two prisms. But in the two prism arrangement refraction occurs when going from the first prism to air and from air to the second prism. And what this effectively does it take the diverging rays coming out of the first prism and make them diverging. But this doesn't happen in a rectangular slab of glass.
  8. Oh sorry I didn't see your post. So basically the shift is so small that the lights just merge together for us?
  9. Drakkith

    Staff: Mentor

    I believe so. If you make your slit REALLY narrow you MIGHT be able to see a spectrum. If you have a rectangular fishtank around you might be able to test it, as the thicker the medium is the larger the shift will be.
    Fill it up, cover the side except for a small entrance slit and shine a light through it in the dark.
  10. sophiecentaur

    sophiecentaur 13,621
    Science Advisor
    Gold Member

    Keeerecktomundo. A thick parallel sided fish tank is a good way to see this sort of thing. (You don't usually get thick enough glass on its own.)
  11. I don't have a fishtank so I can't do that. But I'm happy to know that I was correct about it :biggrin: For once :tongue2:
  12. sophiecentaur

    sophiecentaur 13,621
    Science Advisor
    Gold Member

    Next time you go to an Indian or Chinese takeaway, see if they have a fishtank (very popular in UK takeaways) and look at the dispersion of light from the edges of light sources behind the tank (it needn't be a slit- you can just look at an edge for the effect to show itself). It's nice to have something to do whilst you're waiting but you may have to explain your weird behaviour by giving a Physics lecture to the assembled customers. :wink:

    A pet / aquarian shop would do.
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