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Homework Help: Thin film interference

  1. Aug 25, 2008 #1
    1. The problem statement, all variables and given/known data
    An oil film (index of refraction no = 1.48) floating on water (index of refraction nw = 1.33) is illuminated from above by white light at normal incidence. The film is 360.0 nm thick.

    What is the longest visible wavelength that will be strongly reflected?

    What is the longest visible wavelength that will be strongly transmitted?
    2. Relevant equations

    2nt = (m + 1/2)λ--constructive
    2tn = m λ, with m = 0,1,2,...

    3. The attempt at a solution
    since visible range=400 - 700 nanometers
    the longest visible wavelength for constructive interference ...should it be λ=2t/(2+1/2)
    and so for the second question i use the second equation?
    Last edited: Aug 25, 2008
  2. jcsd
  3. Aug 25, 2008 #2


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    Gold Member

    Do you know the equation for reflectance? I think if the thickness of the film is 1/4λ then the reflectance is minimum.
  4. Aug 25, 2008 #3
    R= ((n2-n1)/(n2+n1))^2
    is it this one?
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