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Homework Help: Oil film thickness for total reflection 500nm

  1. Nov 20, 2011 #1
    1. The problem statement, all variables and given/known data
    I was asked to determin the film thickness for an oil film (n=1.33) for a light beam (500 nm) to give total reflection. I could calculate refraction angle for air to oil, but have no idea as to how to approach this question.

    2. Relevant equations

    3. The attempt at a solution
  2. jcsd
  3. Nov 20, 2011 #2
    This is an interference question. Light reflected from the front surface of the film interferes with light reflected from the back surface. For total reflection the 2 reflected waves must be in phase.
    Points to consider
    1) at the air/film surface a phase change of π (λ/2) occurs (there is no phase change at the film/air surface)
    2) In the film the light travels a distance = 2 x thickness of the film and the wavelength changes.

    Hope this gets you started !!
  4. Nov 21, 2011 #3
    Thank you VERY much. I never thought of it as an interference problem!
  5. Nov 21, 2011 #4
    Its a pleasure. Did you get 94nm (λ/4 for the wave in the oil)?
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