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Homework Help: Index of refraction of different wavelengths

  1. Dec 13, 2011 #1
    1. The problem statement, all variables and given/known data

    Monochromatic light of variable wavelength is incident normally on a thin sheet of plastic film in air. The reflected light is a maximum only for 482.9 and 676.0 in the visible spectrum. What is the minimum thickness of the film ?

    2. Relevant equations
    2nt=m*lambda 2nt=(m+1/2)*lambda

    3. The attempt at a solution
    Ive attemped using both wave lengths in the equation doesnt work.

    Answer is 525nm.
    1. The problem statement, all variables and given/known data

    2. Relevant equations

    3. The attempt at a solution
  2. jcsd
  3. Dec 14, 2011 #2
    This effect is caused by interference between light reflected from the front surface of the film (the air/film surface) and the back surface of the film (film/air surface)
    The first reflection introduces a phase shift of pi.
    The light passing into the film introduces a path difference.
    Hope this shows you how to proceed
  4. Dec 14, 2011 #3


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    Homework Helper

    The order of interference (m) for both wavelengths differ by 1. But you can not get the thickness without the refractive index. Is it n=1.61?

    The reflected light is maximum if

    2tn=(m+1/2)λ,and this is true for two wavelengths =>

    Subtracting the equations we get that 2tn(1/λ1-1/λ2)=1. Solve for t.

    Last edited: Dec 14, 2011
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