1. Not finding help here? Sign up for a free 30min tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Destructive interference in wavelengths question

  1. Jan 22, 2007 #1
    1. A nonreflective coating (n = 1.24) covers the glass (n = 1.52) of a camera lens. Assuming that the coating prevents reflection of yellow-green light (wavelength in vacuum = 564 nm), determine the minimum nonzero thickness that the coating can have.




    2. wavelength of the light in the coating = wavelngth of light / n of coating

    desructive interference: 2t = (1,2...) wavelngth coating
    t=thickness




    3. OK the wavelngth of the light through the coating is: 564/1.24 = 454.84

    I figured maybe that was all i needed and attempted to find t.

    2t=1(454.84)
    t= 227.42

    this was not correct, so I next proceded to find the wavelength once through the film to the glass:

    454.84/1.52 = 299.24 nm

    The destructive interference for this is:

    2t = (1) 299.24
    t= 149.62 nm

    This too was incorrect.

    I even tried plugging in the original wavelength of 564 nm with the n of glass; 1.52, then solving for t. This gave me:

    564/1.52 = 371.05
    2t= (1) 371.05
    t= 185.53


    None of these were correct.

    Any thoughts on where am I going wrong?
     
  2. jcsd
  3. Jan 22, 2007 #2
    I think I've made some progress:

    Since there are 2 1/2 wave shifts then the equation should be:

    2t + 1 wavelength in film = (1/2) wavelength in film

    this gave me an answer of 341.13 nm for t and is still incorrect though.
     
  4. Jan 22, 2007 #3
    Ok nevermind I finally figured it out. I was adding the wave shift across the sides of the formula rather than subtracting it like I should have.

    Thanks to those who considered it for me though. :smile:
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Have something to add?