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Homework Help: Double slit dark fringes

  1. May 5, 2009 #1
    1. The problem statement, all variables and given/known data
    A blue laser beam of wavelength 470nm (in air) is incident on two narrow slits separated by .2mm and produces an interference pattern on a screen located 2m away from the two slits.

    Find the angular position (in degrees) of the 1st dark fringe. Can you use the small angle approximation?

    2. Relevant equations
    tan(theta)= y/L

    3. The attempt at a solution
    I've searched all over for a formula to find the dark fringes, but all i can find is one for the light ones. the dsin(theta)=n(lambda) is used for the maxima. what formula do i use for the minima/dark fringes?
  2. jcsd
  3. May 5, 2009 #2


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    Homework Helper

    For dark fringes use dsin(theta)=(n + 1/2 )(lambda)
  4. May 6, 2009 #3


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    Staff: Mentor

    For two-slit interference, the minima are halfway in between the maxima.
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