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Homework Help: Thin-film interference problem

  1. Sep 28, 2011 #1
    1. The problem statement, all variables and given/known data
    Light of wavelength 470 nm passes through a block of material with a refractive index n = 1.30, some of it is reflected off the air-block boundary and some of it is transmitted through air (n=1.00) with thickness t, and is then reflected off another block also with n = 1.30.

    What is the minimum thickness, t, for which you will observe:

    Constructive interference
    Destructive intereference

    2. Relevant equations

    t = (m+1/2)*lamda / 2n

    t = m*lamda / 2n

    3. The attempt at a solution

    m would be 0 since it's asking for minimum thickness, therefore destructive interference would always be zero wouldn't it?

    I'm a bit confused because another book has the equation:

    t = (m + 1/2)*lamda / 2 <- what happened to the refraction index?
  2. jcsd
  3. Sep 28, 2011 #2


    User Avatar

    Staff: Mentor

    If t was zero there would be no reflection off the second block, and no interference at all! So you'd not have the setup that the problem specifies.

    The refractive index is "bundled" in with the wavelength, lambda; The wavelength used in the equation is the wavelength within the given medium, which depends upon the refractive index.
  4. Sep 28, 2011 #3
    Doh! Massive fail.

    Ah I see what you mean now.

    Thanks a lot!
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