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Diffraction of light question

  1. May 7, 2008 #1
    1. The problem statement, all variables and given/known data
    How many orders of diffraction will be visible if light of wavelength 500nm fals on a grating with 600 000 lines per metre?

    2. Relevant equations

    n = dsinѲ / λ

    3. The attempt at a solution

    n = 6X10⁵m ÷ 5 X 10ˉ⁷m = 1.2
    So 1 order of diffraction will be visible is this right??

    I don't have the angle so I could put it into the equation.
    Last edited: May 7, 2008
  2. jcsd
  3. May 7, 2008 #2


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    Note that d is really (1/600,000) m, so that 600,000 lines would fit in 1 m.
  4. May 7, 2008 #3
    Another way to pose this question is: how many maxima do you see between theta=0 and theta=90 degrees?

    Consider that for a maximum to appear on the wall or the screen in front of the grating the angle theta can never be more than 90 degrees. (Recall that sin(theta) ~ (distance of nth maxima from the 0th maxima)/(distance of the grating from the screen).)
  5. May 8, 2008 #4
    Thanks for your help. Here is my improved answer:
    λ = 500nm = 5 X 10ˉ⁷m
    d = 6 X 10⁵ per metre = 1 ÷ 6 X 10⁵ = 16.7 X 10ˉ⁷m
    n = 16.7 X 10ˉ⁷ X sin 90 ÷ 5 X 10ˉ⁷ = 3.34
    There will be 3 orders of diffraction visible

    Is this correct? If not please tell me where I am going wrong

  6. May 8, 2008 #5


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    Looks good. Problem solved :smile:
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