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Diffraction grating of laser light

  1. Apr 4, 2013 #1
    1. The problem statement, all variables and given/known data
    a narrow beam of laser light (i.e coherent monochromatic light) wavelength 630nm is incident on a grating having 300 lines per mm. a piece of paper is curved 180 degrees beyond the grating. calculate how many spots of red light should be seen

    2. Relevant equations
    d = spacing between lines
    n = order of angle

    3. The attempt at a solution
    so theres 300 lines per mm and i want to find out d therefore i did [itex]\frac{1}{300000}[/itex] and got 3.33x10^-6 lines per m
    so now i can use n*630x10^-9=3.33x10^-6*sin(90)
    this makes

    this gives me 4.72 which I can round up to 5 and multiply by 2 to get the order for 180 degrees.
    so I get an answer of 10 and the books answer is 11.
    what am I doing wrong

    P.S first time using latex...
  2. jcsd
  3. Apr 4, 2013 #2


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    Staff: Mentor

    This should be meter per line I think.

    You forgot the central spot at an angle of zero.
  4. Apr 4, 2013 #3
    check your calculation, I got n to be just greater than 5. As mfb says, you forgot the central max (n = 0)
    If n was 4.72 then you would only see 9 spots (including the central max)
    You cannot 'round up' the value of n !!!
  5. Apr 4, 2013 #4
    you are correct that should have been meter per line. I knew I must have been forgetting something.
  6. Apr 4, 2013 #5
    technician, after seeing your reply I have noticed my calculator is still in radians from my math revision. now i get 5.29 down to 5. Thanks lol
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