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Homework Help: Diffraction Grating Angle Between Second Order Maxima

  1. May 21, 2013 #1
    Hi Everbody,

    I am having a bit of trouble with an AS Physics question regarding diffraction gratings. I have managed to solve the problem that I have been facing, although I am not completely sure that I got to it through the correct means, and also why the answer is such.

    1. The problem statement, all variables and given/known data

    “1. Blue and red light with wavelengths 450nm and 650nm respectively, is inident normally on a diffraction grating which has 4.0 * 105 lines per meter

    b) Calculate the angle between the second-order maxima for these wavelengths.”

    2. Relevant equations

    nλ = d sin θ


    λ = 6.5 * 10-6 and 4.0 * 10-6


    3. The attempt at a solution

    I attempted the solution two different ways with the first, and to me most intuitive method, failing.

    1. sin-1 (2*4.5*10-6)/(2.5*10-6) =θ


    This was incorrect. However, the following corresponded to the answer at the back of the book.

    sin-1 (4.5*10-6)/(2.5*10-6) =θ


    Now the answer at the back of the book was 10.2°, but I assume that it isn’t the result of sheer chance.

    Thank you very much for your time.
  2. jcsd
  3. May 21, 2013 #2
    I managed to solve it I think. It turns out that it was a coincidence.

    The correct method seems to have been:

    (sin-1(2*6.5*10 -7 )/(2.5*10 -6)) - (sin -1 (2*4.5*10 -7)/(2.5*10-6 )) = 10.23°
    Last edited: May 21, 2013
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