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Homework Help: Double-Slit Experiment problem?

  1. May 4, 2016 #1
    1. The problem statement, all variables and given/known data
    In a double-slit experiment, the third order bright fringe is 15 mm from the central fringe. What is the distance of the first (zero-th order) dark fringe from the central maximum?

    2. Relevant equations
    (m+.5)(lambda) = dsin(theta) => dark fringe
    m(lambda) = dsin(theta) => bright fringe

    3. The attempt at a solution
    I honestly don't know. Like, there's no angle or wavelength. The most I've come up with is 15 mm divided by 3 (because it's the third) and getting 5 mm. I seriously don't know.
  2. jcsd
  3. May 4, 2016 #2
    the equation you need is
    dsinθ = nλ for maxima....you have this in your post
  4. May 4, 2016 #3
    But where do I get the angle and wavelength from?
  5. May 4, 2016 #4
    For angles this small you can use the approximation: sin theta = theta = x / D where x
    is the fringe displacement from the central maximum and D is the distance to the screen.
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