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Homework Help: I can't figure out what I am doing wrong Getting Frustrated

  1. Mar 18, 2008 #1
    I can't figure out what I am doing wrong!!! Getting Frustrated!!

    1. The problem statement, all variables and given/known data
    A helium-neon laser (λ=629 nm) illuminates a diffraction grating. The distance between the two m=3 bright fringes is 33 cm on a screen 1.72 m behind the grating. What is the spacing between slits of the grating?

    2. Relevant equations
    Lamda = 629 nm
    m = 3
    y = 0.33 m
    L = 1.72 m
    d = ?

    3. The attempt at a solution
    What I did was tan theta = y/L = 0.33/1.72 = 1086 degrees
    then I used the equation d*sin theta = m*lamda
    and solved for d.... d = m*lamda/sin theta = 1.00x10^-5 m but I am getting it wrong...what am I not doing right???
  2. jcsd
  3. Mar 18, 2008 #2


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    Homework Helper

    If I'm understanding this problem statement correctly, it is giving you the distance between the m=3 maxima on either side of the midline. This means that each of these fringes is half of 10.86º away from the midline, or 5.43º.

    So I think this may be the angle you are to use in the diffraction equation, with m = 3 and lambda = 629·10^-9 m.
  4. Mar 18, 2008 #3
    oh ok thank you thats it!!
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