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Double Slit: Find Longest Wavelength

  1. Jan 18, 2006 #1
    Question: Calculate the longest wavelength of light falling on double slits separated by 1200nm for which there is a first-order maximum. In what part of the spectrum is the light?

    Answer in Book: 12nm

    my problem: the formula i thought you would use is sin(theta) = m(wavelength)/d?? However you do not know an angle? any ideas??
  2. jcsd
  3. Jan 18, 2006 #2


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    Of course [tex]-\frac{\pi}{2} < \theta < \frac{\pi}{2}[/tex]. Can you solve it now?
  4. Jan 18, 2006 #3
    yeah i get what that means and that theta could not be greater then 90 or less then -90, so then how do i incorperate that into the equation? and get 12 nm?:confused:
  5. Jan 18, 2006 #4


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    Plugging theta in would give

    [tex]\lambda _{max} \approx d[/tex]

    Which according to your book is wrong. I'm stunned. I suppose the answer in the book is wrong: they've forgotten the zeros.
  6. Jan 18, 2006 #5
    so the biggest possible wavelength that could be created would be slightly under the width between the two slits?? and thanx a lot for your help:smile:
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