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Thin film interference formulae

  1. Aug 1, 2011 #1
    Hello, I am trying to understand thin film (in air) interference but I have a problem:

    I know we have destructive interference when [itex]\delta=(2m+1)\pi[/itex].

    Now i can try to calculate the thickness of the film to get it, so

    since [itex]\delta =\frac{4nt\pi}{\lambda} - \pi[/itex] where t is the thickness,

    I come to [itex]\delta=(2m+1)\pi=\frac{4nt\pi}{\lambda} - \pi [/itex]

    that leads me to [itex] t=\frac{\lambda(m+1)}{2n} [/itex] instead of
    [itex] t=\frac{m\lambda}{2n} [/itex]

    (n is the refractive index and m is the integer parameter)

    Where am I wrong?
  2. jcsd
  3. Aug 1, 2011 #2

    Doc Al

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    Staff: Mentor

    Why do you think you're wrong?
  4. Aug 1, 2011 #3
    Thank you for your answer, I think I am wrong because my book's solution is [itex] t=\frac{m\lambda}{2n}[/itex]
  5. Aug 1, 2011 #4

    Doc Al

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    Staff: Mentor

    The answers are essentially equivalent. (You can always start with m = -1 in your version.)
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