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Homework Help: The thinnest film in which the reflected light will be a max

  1. Dec 18, 2016 #1
    1. The problem statement, all variables and given/known data
    Monochromatic light, at normal incidence, strikes a thin film in air. If λ denotes the wavelength in the film, what is the thinnest film in which the reflected light will be a maximum?

    • A


    • B

    • C

    • D

      much less than λ

    • E


    2. Relevant equations
    n air ∠ n glass
    so at the first reflection it will cuase a 180° ray out of phase while the second ray refected from the inside it will stay inphase because n air ∠ n glass
    so for constructive interference: 2L=odd number*λ/2
    3. The attempt at a solution
    least odd number is one , so L=λ/4 ?? why it is incorrect
    Last edited by a moderator: Apr 18, 2017
  2. jcsd
  3. Dec 18, 2016 #2


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    Homework Helper
    Gold Member
    2017 Award

    Your answer of λ/4 is correct. The problem statement does not mention glass.
  4. Dec 18, 2016 #3
    Well all I can do is sympathize. 1/4 should have been correct. Perhaps they have in mind a low index film on a high index substrate? Or perhaps the problem setter forgot about the 180 phase shift front to back and just got it wrong.
  5. Dec 19, 2016 #4


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    Staff: Mentor

    possibly intended
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