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Diffraction orders seen

  1. Jul 21, 2010 #1
    1. Light of wavelength 632.8nm is normally incident upon a grating of 5000 lines/cm. How many different diffraction orders can be seen in transmission?

    2. m(lambda)=d(sintheta)

    3. According to my text, d=1/5000lines/cm = 2x10^-6m
    and sintheta= lambda/d = 632.8x10^-9/ 2x10^6 = .3164
    If I take sin^-1(.3164), I get 18 degrees.
    From here, I don't know what to do because the answer is that 7 beams exist, corresponding to 4 orders, and I don't know how to get to that answer!
  2. jcsd
  3. Jul 21, 2010 #2


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    sinθ = mλ/d.

    λ/d = 0.3164. sinθ cannot be more than 1. So m should be 3. Hence there are 3 beams on either side and one central beam.
  4. Jul 22, 2010 #3
    Thanks for your reply. Although I understand what you are saying about the 3 beams on each side with one central, how do you derive that explanation from 0.3164?
  5. Jul 22, 2010 #4


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    sinθ = nλ/d. if n = 3
    sinθ = 3*λ/d = 3*0.3164 = 0.9492< 1. So n cannot be more than 3.
  6. Jul 22, 2010 #5
    Thanks, I understand now.
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