Double Slit: Find Longest Wavelength

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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??
 

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  • #2
Päällikkö
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Of course [tex]-\frac{\pi}{2} < \theta < \frac{\pi}{2}[/tex]. Can you solve it now?
 
  • #3
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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:
 
  • #4
Päällikkö
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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.
 
  • #5
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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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