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Optics and thin film interference question.

  1. Feb 20, 2009 #1
    1. The problem statement, all variables and given/known data
    The thin film is of two glass with air in the middle.
    refractive index of...
    air: n=1.0003
    glass: n=1.5

    Q1.) Calculate optimum thickness of the film (for 1st order interference)

    Q2.) Why does the distance between interference fringe beome longer (increased fringe spacing) when the film thickness is reduced?

    2. Relevant equations

    We aren't given any, we have to find them ourselves.
    I found:
    and 2t=(m+1/2)(lambda/n)
    one is constructive one is destructive, not sure which is which for this case.
    not sure if they are even relevant here.

    3. The attempt at a solution

    Q1.) I found: t=m(lambda) / 2n
    and used that to get 3.19 x10^-7 m
    Is that correct?

    Q2.) I drew a diagram that explains it, and I think it has to do with missing fringes in between due to destructive interference, however I don't have any equation that shows that.
    I think this is relevant: 2t=m(lamba/n)
    I think that's for constructive interference. t is directly proportional to m (# of visible fringes). When t is decreased, the # of fringes decrease. Thus, providing empty spaces between fringes. But how do I prove that it's every other fringe that disappears? Is there any equation that shows thickness is inversely proportional to fringe spacing?
  2. jcsd
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