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Homework Help: Thin Film interference

  1. Nov 26, 2007 #1
    A thin film is put on the surface of glass, with indexes of refraction of x and y respectively.

    If light, with wavelength L in air, is reflected from the 2 sides of the film, to interfere constructively, what is the minimum thickness required.

    The question is stated just like that, so I think it is vague in some parts.

    I can't think of a formula to work this out. I know for con. interference, 2t=mL, where m is a whole number. But this isn't enough, and doesn't relate the indexes of r.

    Any hints????
  2. jcsd
  3. Nov 26, 2007 #2
    Shouldn't be doing your Homework 3 and half hours before its due.. tut tut:tongue2:
  4. Nov 26, 2007 #3
    Haha. I'd usually do it on Saturday, but with a boozin session going down, it didnt get done.

    Besides, I'm a member here ages, so the guys can be great help when needed.

    So did you get this one???? Hey I helped you with the polarizer one :approve:
  5. Nov 26, 2007 #4
    Sure i got it......................

    Nah i think i'm retarded, on 40%..

    Still can't get the polarizer one or this one or the last one lol
  6. Nov 26, 2007 #5
    Ah....there would have been a nice a pav pint in it for ya.

    Hey, what's your name? Don't tell me your TP.....:rolleyes:
  7. Nov 26, 2007 #6
    Im taking a break, think "Fringes from Different Interfering Wavelengths" may be answerable! If i hit 60 i'll be proud! lol ;)
  8. Nov 26, 2007 #7
    I don't think I'll hit 60 this time....

    Damn weekends....;)
  9. Nov 26, 2007 #8

    Doc Al

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    Staff: Mentor

    Assuming x < y, the phase change on reflection is the same for both reflections so all that matters is the phase difference due to traveling through the film. That equation (2t = mL) will work, but L must be the wavelength in the film. That's where the index of refraction comes in.

    If x > y, things will be a bit different. Learn more here: http://hyperphysics.phy-astr.gsu.edu/hbase/phyopt/thinfilm.html" [Broken]
    Last edited by a moderator: May 3, 2017
  10. Nov 26, 2007 #9
    Yes, x > y.

    I understand it when x < y.

    Then 2t=L, where L is wavelength in the film. So L would be index of refraction in film times wavelength in air.

    But x > y.....
  11. Nov 26, 2007 #10

    Doc Al

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    Staff: Mentor

    Not exactly. Is the wavelength shorter or longer in the film?

    In that case the second reflection undergoes a different phase shift than the first (actually no phase shift). See that link for details.
  12. Nov 26, 2007 #11
    Sorry, divided by.....

    So does that mean 2t=L?? Or t=L?
  13. Nov 26, 2007 #12

    Doc Al

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    Staff: Mentor

    When x > y, the condition for constructive interference (and minimum thickness) is 2t = L/2 (where L = wavelength in the film).
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