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Diffraction Barrier-Class of Maximum?

  1. Jun 14, 2017 #1
    1. The problem statement, all variables and given/known data

    A Diffraction Barrier has 4200 openings per 1cm. A screen stands 2.00 m opposite of the Barrier. Say that for a certain class m, the maximums that correspond with two different wavelengths (589.00 nm & 589.6 nm) abstain from each other by 1.54 mm. What is the value of m?

    2. Relevant equations

    tanθ = y/L
    sinθ = m*λ/d

    3. The attempt at a solution

    Alright, we have:

    λ1 = 589.0 nm
    λ2 = 589.6 nm

    λ2 > λ1 => y2 > y1 => D = 1.54 mm = y2 - y1

    tanθ ~ sinθ [θ is a small angle)

    Also, d = 1 cm/4200 = 2.38 * 10-4m


    D = L*m*(λ2 - λ1)/d <=> m = d*D/(λ2 - λ1)*L <=> ... <=> m = 3

    My problem is that the naswer the book gives is m = 2. I run the numbers many times, and I always get 3. Am I doing something wrong? Is there a problem in the math or the logic?

    Any help is appreciated!
    Last edited: Jun 14, 2017
  2. jcsd
  3. Jun 14, 2017 #2


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    I find 3 just like you do. Looks as if book answer is wrong.
  4. Jun 14, 2017 #3
    It's been wrong before, but I wanted to make sure. Thanks a lot for the input!
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