When diffracting light through a double slit, I understand that the two sources must arrive on the screen m(wavelengths) out of phase [where m is any whole number], in order for a bright fringe to form. This is due to the fact that a path difference of mλ would result in constructive interference, i.e. reinforcement, and hence maximum intensity would be detected at this point.
However, when diffracting light through a single slit, the situation is different. The two sources must arrive on the screen (m+1/2)λs out of phase, e.g., λ/2, 3λ/2, 5λ/2 etc, for constructive interference. I cannot understand why this is the case, and my physics teacher cannot either. Why would this result in constructive interference and not destructive?
Any help greatly appreciated!