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Destructive Interference and path difference

  1. Jan 21, 2008 #1

    I hope I'm posting in the right place!

    In the destructive interference of waves and solving for the path difference, I"m confused at when we should use m+1/2 vs. m-1/2 ( so the eq'n is PD=(m+1/2)lambda or PD=(m-1/2) lambda

    Any insight would be greatly appreciated! :)

  2. jcsd
  3. Jan 21, 2008 #2
    m is a number of how many dark spots away you are from the center. The sign determines which direction on the interference pattern from the center is your dark spot.
  4. Jan 21, 2008 #3

    Doc Al

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    Staff: Mentor

    If you're talking about double slit interference, then destructive interference (dark fringes) happen when the path difference is a 1/2, 3/2, 5/2, etc., multiple of the wavelength. So you can use PD=(m-1/2)lambda, where m stands for the number of the dark fringe (m = 1, 2, 3, etc.; first, second, third, etc., dark fringe) counting from the central maximum.
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