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Diffraction problem about calculating the image on the screen

  1. Feb 12, 2012 #1
    1. The problem statement, all variables and given/known data
    http://pds22.egloos.com/pds/201202/12/94/f0095594_4f3768f25c489.png [Broken]

    2. Relevant equations

    3. The attempt at a solution
    f=0.25m (focal length)

    θ=1.22λ/b=(1.22*560*10-9)/0.03=2.3*10-5 (rad)

    θ=tan(d/f)=d/f since d/f is close to 0.
    Thus, d=fθ=5.7*10-6m
    Diameter 2d=1.1*10-5m

    Text book had the same walkthrough up to θ=2.3*10-5 (rad),
    but then it did as following
    2 * 3.0 * 10-2 * 2.3 * 10-5 = 1.4*10-6mm
    which implies that d=bθ, but that doesn't make sense to me.
    Could you please help me? :(
    Last edited by a moderator: May 5, 2017
  2. jcsd
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