Physics Help Forum Need explanation for Interference formulas

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 May 2nd 2010, 02:25 PM #1 Junior Member   Join Date: Mar 2010 Posts: 3 Need explanation for Interference formulas I'm rather confused about why the destructive interference formula is (d)sin(theta) = (m + 1/2)(lamda), m=0,1,2,3... instead of (d)sin(theta) = (m - 1/2)(lamda), m=1,2,3... Unless I'm misunderstanding something, m = to the order. So the first order bright fringe is m=1. However, the first order dark fringe, if you use m=1, you get the wrong answer, because it should be (d)sin(theta)=(0+1/2)(lambda), not (d)sin(theta)=(1+1/2)(lambda) (which is the second-order dark fringe). Is m NOT the same as the order? Because if it was, the formulas dor destructive interference doesn't work. Mind someone explain this to me? Thanks in advance. Last edited by EricOu; May 2nd 2010 at 02:28 PM.
 May 2nd 2010, 07:55 PM #2 Senior Member   Join Date: Apr 2008 Location: HK Posts: 886 m is just an arbitrary number. It may not represent the order of the dark fringe. for (m - 1/2)(lamda), m=1,2,3... m is the order of dark fringe for (m + 1/2)(lamda), m=0,1,2,3... this m is not the order of dark fringe but just a non-negative integer __________________ Good results were achieved and the new task is to become a good doctor.