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Diffraction gratings and young's double slit experiment

  1. Mar 19, 2014 #1
    Just a question about the naming of optical phenomena.

    I've taken these explanations from a book I'm using:

    Diffraction is when a single wave passes a slit or obstacle and spreads out. The single wave produces a diffraction pattern.

    Interference is when we have more than one source of coherent waves which combine together at a given point. Their phase difference determines the intensity at that point.

    These definitions make sense in terms of the interference pattern formed in Young's double slit experiment, and the diffraction pattern seen from single slit diffraction. However, what about a diffraction grating - the above seem to imply it is an interference phenomenon producing the lines.

    Can anybody please explain the idea behind this naming - or is it a good idea to not worry too much about the names? Thanks.
  2. jcsd
  3. Mar 20, 2014 #2

    Simon Bridge

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    Not quite - it is possible for a wave to pass through a wide slit with no (discernable) diffraction pattern forming - like a water wave passing into a harbour.
    You also get diffraction from multiple sources and no slits.

    Diffraction occurs as a result of interference from multiple wave sources.

    Almost - parts of a single wave can interfere with other parts - i.e. if it passes through a slit or slits or is reflected.

    They all produce interference - diffraction can occur from multiple sources.

    The names are mired in history - it is a good idea not to worry about the names: concentrate on the phenomena.

    diffraction: from "to break apart".

    interference: from "interfere" - in the sense of subtraction or difference.
  4. Mar 20, 2014 #3
    Thanks for your reply.

    So in the three scenarios I mentioned (single slit diffraction, Young's double slits and a diffraction grating), we have interference going on.

    You said 'Diffraction occurs as a result of interference from multiple wave sources' which explains the two first and third above. In the first case the Huygen's wavelets make up the multiple sources, and in the third we have many slits making up the sources.

    In terms of double slit interference however, don't we still have multiple wave sources - why then is this not classed as diffraction?
  5. Mar 20, 2014 #4

    Simon Bridge

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    That's right.

    At this stage just be aware that this is the wave model for diffraction and interference - there is also a particle model. Worry about the details later - get these ideas down first. Just understand that what you are learning now is not the last word.

    It is classed as [an example of] diffraction.

    Compare the pics below - starting with one slit (top) adding a second one makes the pattern brighter but with bits "missing".


    This "removal" is what was originally meant by "interference".
    ... it is as if adding a second slit interfered with the normal 1-slit pattern.

    You are still worrying too much about the exact names.
    The use of words changes with time so we get stuck with names for things that don't always make a lot of sense.

    The above is called "Young's Interference" because of history - it is equally acurate to call it "double-slit diffraction" to show it is an example of the class of phenomena "diffraction at slits".
    Last edited: Mar 20, 2014
  6. Mar 21, 2014 #5
    Ok, I got it, thanks! :)
  7. Mar 22, 2014 #6

    Simon Bridge

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    No worries...
    And don't sweat the terminology ;)
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