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Old May 2nd 2010, 02:25 PM   #1
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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.
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Old May 2nd 2010, 07:55 PM   #2
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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
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