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Dispersion/Refraction, but WHY?

  1. Sep 15, 2009 #1
    I understand and have seen light disperse as it shines through a prism, crysal or other medium; I have seen snells law, I have searched this forum for answers..!

    But I can not find an explanation I can understand of *why* a shorter wavelength of light will bend differently to a longer wavelength? What is the nature of the interaction or the nature of the wave that causes the bend, and why would the wavelength be the determining factor?

    Thanks in advance for the newb-busting answer!
    Matt
     
  2. jcsd
  3. Sep 15, 2009 #2

    tiny-tim

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    Hi Matt! :smile:

    It's the speed of the light that determines it …

    the index of refraction is the ratio of the speeds before and after …

    and the speed happens to depend on the wavelength.

    (why? well, that's quantum mechanics for you! … something to do with the way the light interacts with the molecules. :redface:)
     
  4. Sep 15, 2009 #3
    Thanks Tim. You've sort of highlighted the question... I have access to plenty of text that tells me what will happen, but nothing I've found that tells me why?
     
  5. Sep 15, 2009 #4
    The why is usually harder than the what.

    EDIT: I was looking for some link to give you a better intuitive idea of what is going on when a wave interacts with the 'boundary' but I can't find anything!
     
    Last edited: Sep 15, 2009
  6. Sep 16, 2009 #5

    Andy Resnick

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    Ultimately, Physics cannot answer 'why'. We sometimes describe observations and sometimes make predictions. We observe dispersion, we have reasonably good models, and we can predict the dispersion for some simple materials. That's as good as it gets.
     
  7. Sep 16, 2009 #6

    morrobay

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    At the microscale an electromagnetic wave is slowed in a material because the electric field
    creates a disturbance in the charges of each atom - primarily the electron proportional to the permittivity.
    The oscillation of charges itself causes the radiation of an electromagnetic wave that is
    slightly out of phase with the original wave. The sum of the two waves creates a wave of
    the same frequency but shorter wavelength than the original leading to slowing of the waves travel.
    * An electron in an atom or molecule is bound there by strong restoring forces.It has a
    definite natural frequency.For electrons in atoms it is usually in a region corresponding to
    violet or U.V. light. In mechanical systems it is possible to "drive" the system most
    effectively if we impress on it an external force whose frequency is as close as possible to that of the natural resonate frequency.In the case of light the blue is closer to the natural
    resonate frequency of the bound electrons than red light. Therefore we would expect the blue light to be more effective in causing the electrons to oscillate.

    last paragraph referenced from Physics, Halliday-Resnick
     
    Last edited: Sep 17, 2009
  8. Sep 16, 2009 #7

    jtbell

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    I guess you haven't checked out the Physics Forums FAQ at the top of this very forum? :smile:

    (in particular post #4)
     
  9. Sep 17, 2009 #8
    I have - and it's gives a superb explanation of the change in speed; but I can't see anything that explains why the angle of refraction changes in relation to the wavelength

    Any pointers to layman-friendly sources would be gratefully receieved!

    Thanks
    Matt
     
  10. Sep 17, 2009 #9

    Doc Al

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    What matters is the change in speed, which of course changes the wavelength. When a wave encounters a medium (of higher index of refraction) it slows down. If it meets the boundary at an angle, the change in speed will cause different parts of the wave to slow down at different times, thus changing the direction of the wave. See: http://www.physicsclassroom.com/Class/refrn/U14L1e.cfm" [Broken]
     
    Last edited by a moderator: May 4, 2017
  11. Sep 17, 2009 #10

    Andy Resnick

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    I relaize this is a stock answer, but possibly confusing to the student- there is a steady stream of 'how does one part of the wave know what the other part is doing?' type questions here.

    Better (IMO) to simply say that the momentum changes, so the direction changes.
     
    Last edited by a moderator: May 4, 2017
  12. Sep 17, 2009 #11

    Doc Al

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    You're right, there's a good bit of handwaving here. But I think the argument can be made at least semi-rigorous by invoking Huygen's principle and detailing how wavefronts are defined.
    Can you expand on this?
     
  13. Sep 17, 2009 #12
    Morrowbay's post #6, including the quote from Halladay & Resnik, describes the dispersion caused by the atomic electric dipole resonances and anomalous dispersion (see http://en.wikipedia.org/wiki/Dispersion_(optics [Broken])
    usually in the UV region, where the index of refraction, which normally increases with decreasing wavelength, suddenly increases very fast, and then decreases very fast (the anomalous part), diving below n=1. The phase shift described by morrowbay is identical to the phase shift observed in RLC electric circuits, when driven off-resonance.
    Bob S
    [Edit] See also discussion in Section 7.5 in Jackson "Classical Electrodynamics" (second edition) on frequency dispersion.
     
    Last edited by a moderator: May 4, 2017
  14. Sep 17, 2009 #13

    Andy Resnick

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    Last edited by a moderator: Apr 24, 2017
  15. Sep 17, 2009 #14

    Doc Al

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    Hey, I like it. Thanks, Andy!
     
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