Diffraction orders seen

  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. rl.bhat

    rl.bhat 4,433
    Homework Helper

    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. 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. rl.bhat

    rl.bhat 4,433
    Homework Helper

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