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Diffraction and interference

  1. Mar 24, 2011 #1

    could someone tell me what is the main difference between effects of diffraction and interference?

    For now, I think that diffraction is name for deviation of waves when they come to obstacle.
    And interference is phenomenon when two or more waves are in superposition in some point in space.

    But the main thing that I do not understand is the difference between diffraction and interference picture. I simply think that those two are the same, because, first wave has to do diffraction on some obstacle and after that to interferes with itself.

  2. jcsd
  3. Mar 24, 2011 #2
    I think diffraction is the term used when interference is caused by multiple sources that are being driven by a single source.
  4. Mar 24, 2011 #3


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    Staff: Mentor

    A wave can diffract without interfering with itself. They are seperate phenomena that you have already defined in your post. :biggrin:
  5. Mar 24, 2011 #4
    Diffraction and interference are basically the same, but the former is interference of (uncountably) infinitely many waves and the latter finite.
  6. Mar 25, 2011 #5
    Well, ok if diffraction is term for deviation of waves when they come to the obstacle, then there can not be such thing as diffraction picture, right? It is interference picture, because there is superposition of waves in some point in space, but before that diffraction happened.

    I understand all of physics of waves, but I do not know why do they have term "diffraction picture" when it obviously comes from interference in last step (but still can't happen without diffraction).

    From physical point of view they could call Fraunhofer diffraction, something like Fraunhofer interference? And same with Fresnel.

    In fact, I would like to ask what is the difference between diffraction picture and interference picture?
  7. Mar 25, 2011 #6


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    Staff: Mentor

    The phenomena that we commonly label as "interference" (e.g. of light passing through two slits) and "diffraction" (e.g. of light passing through one slit) are both the result of superposition / interference of waves.

    We commonly use "interference" to refer to superposition of waves from a discrete number of sources, e.g. two narrow slits, three narrow slits, or a diffraction grating. We commonly use "diffraction" to refer to superposition of waves from a continuous set of sources, e.g. the continuous range of points across the width of a wide slit.

    Two narrow slits gives you a pure interference pattern. A single wide slit gives you a pure diffraction pattern. Two wide slits gives you a combination of diffraction and interference.

    Double slit interference

    Single slit diffraction

    Double slit diffraction (The "single slit envelope" is the "diffraction" part of the pattern, and the narrower peaks are the "interference" part.)
  8. Mar 25, 2011 #7
    @ jtbell

    Thank you very much, I finally understood the main difference between interference and diffraction phenomena :) .
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