In this case, the two slits act as a source of waves that are in phase. Hence, a maxima is produced on a screen when the path difference between the waves that arrived at that point is an integer number of wavelengths (constructive interference). A minima is produced when the path difference is equal to an odd-number of half-wavelengths (destructive interference). 

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If instead the slits acted as a source of waves that are in anti-phase, then the conditions of constructive and destructive interference will reverse. I.e., a maxima is produced when the path difference is equal to odd-number of half-wavelengths (constructive interference) and a minima is produced when the path difference is an integer number of wavelengths (destructive interference).

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