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How to calculate slit separation?

  1. Feb 23, 2016 #1
    1. The problem statement, all variables and given/known data
    Light of wavelength 680 nm falls on two slits and produces an interference pattern in which the fourth order fringe is 38 mm from the central fringe on a screen 2.0 m away. What is the separation of the two slits?

    2. Relevant equations
    d * sin(theta) = m * lambda

    3. The attempt at a solution
    I tried to calculate the slit separation using the equation above but I'm not sure about the angle. Using tan^-1(.038/2), I got an angle of 1.1 degrees which then led me to a d value of 1.41 x 10^-4 m, but the answer given by the book is .085 mm so I'm just completely lost at this point.
  2. jcsd
  3. Feb 23, 2016 #2


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    I also get a similar answer as yours.
  4. Feb 23, 2016 #3
    Perhaps the book is wrong or something then.
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