Hi Guys, my textbook mentioned that how the equation for the minima for single slit diffraction was derived: Consider a slit of width W with 2 light rays, one emitting from the edge, one emitting from the center. Their path difference is W/2 sin T . If the path difference is 1/2 lambda, then they will experience destructive interference. Same can be said for light rays spaced apart by W/3, W/4, so on. Hence the general equation for the minima, W sin T = m (lambda). This is perfectly reasonable. However, I say: Consider 2 light rays emitting from the single slit at each edge. Path difference will be W sin T. If W sin T were an integral multiple of wavelength, there should be constructive interference. So W sin T = m lambda can be the equation for constructive interference too. Why is it that light rays spaced apart by the entire width of the slit aren't counted?