Physics-
General
Easy
Question
Two ideal slits S1 and S2 are at a distance d apart, and illuminated by light of wavelength 
passing through an ideal source slit S placed on the line through S2 as shown. The distance between the planes of slits and the source slit is D. A screen is held at a distance D from the plane of the slits. The minimum value of d for which there is darkness at O is
The correct answer is:
Path difference between the waves reaching at
where
Initial path difference
Path difference between the waves after emerging from slits.
and 
(From Binomial expansion)
For obtaining dark at O, 
must be equals to 
i.e. 
For minimum distance 
so 
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