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Diffraction question

  1. Jul 27, 2010 #1
    Quick question (hopefully)

    Diffraction is often classically described as light bending around corners, Huygen’s Principle treats each point of an advancing wavefront as a new source.

    My grad physics professor has mummbled one time that diffraction occurs due to the interaction of light with the diffraction medium. The electric and magnetic fields perturb charges in the diffractin gratting which cause radiation from those charges due to their acceleration. The diffraction effect occurs due to the interaction with the medium.

    Also, am I correct in saying that Huygen’s Principle can only be applied to diffraction, not light in general (You can't apply it to a laser).

    Thanks in advance

  2. jcsd
  3. Jul 27, 2010 #2

    Andy Resnick

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    Re: diffraction questionnn

    Huygens' principle is universally valid, AFAIK. It is another way of stating Fermat's principle of least time.
  4. Jul 27, 2010 #3
    I think Huygen's principle is very general and applies to lasers. For example, my understanding is that since a laser comes out of an aperture of finite size, it's "narrowness" over long distances is limited by diffraction. A laser coming out of a tiny aperture will spread out fairly quickly, while a laser that comes out of a really wide aperture can stay focused down longer.

    I think you can see this using Huygen's principle. An infinite plane wave would propagate forever without changing shape. Lasers are approximations of plane waves but fall short because of their finite lateral extent. So draw a finite plane wave and spherical wave fronts emanating from it. You'll see that they mostly constructively interfere in the direction of propagation of the wave, so the wave mostly keeps going in that direction. But it also "frays" at the edges from wave fronts spreading laterally, so the wave spreads out over time. You can see that a narrow wave will spread out very quickly, and the opposite for a wide wave (I think this is the same thing as the uncertainty principle).
  5. Jul 27, 2010 #4
    What about my first question??

  6. Jul 27, 2010 #5
    I'm not clear what the claim is there but in the case of, say, diffraction through a set of slits, it's the light that *doesn't* hit any material medium that diffracts out behind the slits.
  7. Jul 27, 2010 #6
    yes, but the its possible for the edges that interact with light to be responsible for diffraction.

    Got this off wikepedia


    "For instance, a travelling EM wave incident on an atomic structure induces oscillation in the atoms of that structure, thereby causing them to emit their own EM waves, emissions which alter the impinging wave through interference. These properties cause various phenomena including refraction and diffraction."

    So I think old Professor Zahed was right.
  8. Jul 27, 2010 #7
  9. Jul 27, 2010 #8

    He was not talking about Bragg diffraction.. he was talking about single & double slit.

    I'm, not saying that it depends on the medium, Im thinking that it is a result of the medium. The incommong electric fields shake the electrons at the same frequency, which produce their own light because their being accelerated. This light however doesn't exactly leave in the same direction it came in. I'm thinking whether you have wood or metal or crack, it will be the same.. just depends on the geometry.. which is the grating.

    But I could be wrong.
  10. Jul 28, 2010 #9

    Andy Resnick

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    One way to 'explain' diffraction is that when a wavefront is truncated (apodized, passed through an aperture, etc), diffraction occurs. Then, diffraction is seen to be an essential property of any non-isotropic source.
  11. Jul 28, 2010 #10


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    Yes that's true. That's actually true of any kind of scattering, diffraction, refraction or any -actiony thing you come up with in classical electromagnetics. There is a principle called the equivalence principle in electromagnetics that allows you to redefine a scattering problem as a homogeneous medium with your original incident field going through unperturbed and a secondary field (the scattered field) being produced by sources in the medium. The sum of the two gives you the total field that you would observe in the actual problem. The idea is that when the incident field impinges on a scatterer it excites conduction and displacement currents on and throughout the scatterer. These currents produce their own electromagnetic waves that are called the scattered field. The superposition of the scattered and incident waves gives you the diffracted, reflected and refracted waves that you would normally observe. And these are not ficticious sources either, the currents that we are talking about are the same currents that are exciting on say a receiving antenna.
  12. Jul 28, 2010 #11
    Thank you!! very clear
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