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Homework Help: Constructive/Destructive Interference Problem.

  1. Feb 6, 2007 #1
    1. The problem statement, all variables and given/known data
    If we shine white light on an air wedge like that shown in the figure below, the colors that are weak in the light reflected from any point along the wedge are strong in the light transmitted through the wedge. Explain why this should be so.
    http://www.practicalphysics.org/imageLibrary/jpeg350/128.jpg [Broken]

    2. Relevant equations
    Constructive reflection from thin film, no relative phase shift

    2t = m*lambda (m = 0,1,2...)

    Destructive reflection from thin film, no relative phase shift

    2t = (m + 0.5)lambda (m = 0,1,2...)

    3. The attempt at a solution
    Since the light is passing through the wedge has a greater index of refraction, that means that the light wave actually slows down as it passes through.

    In waves, v = f * lambda so we can find the relationship between the constructive interference and the velocity of the wave by

    (d/t) = f * lambda
    t = d / (f * lambda)

    2(d / f * lambda) = m lambda

    2d/f = m.

    And I'm completely stuck here. I think I'm going in a wrong direction to answer this problem. Can anyone help me?
    Last edited by a moderator: May 2, 2017
  2. jcsd
  3. Feb 7, 2007 #2
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  5. Feb 8, 2007 #4
  6. Feb 9, 2007 #5

  7. Feb 11, 2007 #6
    someone please help me.........
  8. Feb 11, 2007 #7
    Sigh it's almost been a week, yet no answer
  9. Feb 17, 2007 #8
  10. Feb 17, 2007 #9


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    Homework Helper

    I don't really have to much to say about this off the top of my head, but here's what I would think about:
    This doesn't really have anything to do with the velocity. You need to consider the index of refraction though, to determine if there is a phase shift when the light is reflected at each boundary. This will determine if there is constructive or destructive interference, and where.
  11. Feb 17, 2007 #10
    How exactly do you determine the phase shift to this question?
  12. Feb 17, 2007 #11


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    Homework Helper

    I think the rule is if the light reflects from a medium of a higher index of refraction than that of the medium it is travelling in, you will get a phase change of 180 degrees.
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