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Confused on Constructive/Destructive Interference

  1. Mar 30, 2008 #1
    Confused on Constructive/Destructive Interference!!!

    1. The problem statement, all variables and given/known data
    A film with n=1.6 is deposited on glass. What is the thinnest film that will produce constructive interference in the reflection of light with a wavelength of 575 nm?

    2. Relevant equations

    3. The attempt at a solution
    I have 2 equations that I can choose from...

    Constructive Interference for 0 to 2 refl. phase changes
    Destructive Interference for 1 refl. phase changes
    2T = m * t/n
    t = wavelength

    Destructive Interference for 0 to 2 refl. phase changes
    Constructive Interference for 1 refl. phase changes
    2T = (m + 1/2) * t/n
    t = wavelength

    How do I know which one to use?

    I chose the 2nd one because I assumed there was only 1 phase change.

    How do figure out what m =???
  2. jcsd
  3. Mar 30, 2008 #2
    You can't just assume there's only one phase change. You have to see if the reflected beams of light undergo a hard or soft reflection.

    As for the value of m, you want it to be as small as possible so that the thickness is as small as possible.
  4. Mar 30, 2008 #3
    whats hard and soft? those terms are not in my book.
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