Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

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


    User Avatar
    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
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook