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Homework Help: Double-slit experiment: What is the speed of the neutrons?

  1. May 8, 2016 #1
    1. The problem statement, all variables and given/known data
    An experiment was performed in which neutrons were shot through two slits spaced 0.10 mm apart and detected 3.5 m behind the slits. The following figure shows the detector output. Notice the 100 μm scale on the figure. To two significant figures, what was the speed of the neutrons?
    2. Relevant equations
    Δy = λL/d (spacing between maxima)
    λ = h/mv (de Broglie λ)
    m= 1.67*10^-27 Kg

    3. The attempt at a solution
    The spacing between the maxima appears to be 100*10^-6 m in the figure, so:
    (Δy*d)/L = λ = ((100*10^-6 m)*(0.1*10^-3 m))/3.5 m = 2.85714*10^-9 m = de Broglie λ

    v = h/(mλ) = (6.626*10^-34 J.s)/((1.67*10^-27 Kg)*(2.85714*10^-9 m) = 138.9 m/s

    This answer is incorrect. Can someone please give me a hint as to what I'm doing wrong?
  2. jcsd
  3. May 8, 2016 #2


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    Could you check that once more for me ?

    (oh, and 138.9 is not what I would call 'to two significant figures' ! )
  4. May 8, 2016 #3
    Hum, on second look it looks more like (2/3)(100*10^-6m).

    Which gives Vneutron = 2.1*10^2 m/s; which is the correct answer! Thanks a lot!
  5. May 8, 2016 #4


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    well done!
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