Origin of Fringes from Double-Slit

• Constructive interference happens when path length difference is D = nl

• Destructive interference happens when path length difference is D = (n + 1/2)l.

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The center of a bright fringe corresponds to constructive interference of the light from the two pinholes, which happens when light from the two pinholes are in phase. We know they are in phase and coherent at the pinholes themselves, so they are in phase again if the distance to a distant screen is the same or different by an integer number of wavelengths.

Similarly, there will be destructive interference when the light from the two pinholes are 180 degrees out of phase, or when thepath length difference is an odd number of half wavelengths.