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Fabry Perot Cavity - phase difference?

  1. Apr 13, 2010 #1

    I'm having trouble understanding where the formula (appearing in my notes or Wikipedia) comes from.
    One can read on Wikipedia: The phase difference between each succeeding reflection is given by
    [tex] \delta = \frac{2 \pi}{\lambda}2nlcos\theta [/tex]
    Where n is refractive index inside cavity, l distance between cavity walls, [tex]\theta [/tex] angle of the ray.

    What are we actually finding?
    It would seem we are multiplying wavenumber k by refractive index n and distance traveled by light. But as I look at it, it seems to me that distance traveled is [tex]\frac{2l}{cos\theta} [/tex]. And i don't understand why would we do that to find phase difference.

    Can anyone explain what are we actually doing here?
  2. jcsd
  3. Apr 13, 2010 #2
    nevermind, i found the answer.
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