Two ideal slits S 1 and S 2 are at a distance d apart, and illuminated by light of wavelength λ λ passing through an ideal source slit S placed on the line through S 2 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
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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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