Reading the derivation for the single slit diffraction minima formula from this site, I'm not convinced about its argument. http://www.math.ubc.ca/~cass/courses/m309-03a/m309-projects/krzak/index.html" [Broken] What I'm having trouble understanding is the explanation of the formula for minima when m>1. For example when dealing with m=2, the explanation is that the slit is split into four sections so there is a phase difference of pi for the paired up point sources and so destructive interference occurs. But if we used 2 sections for m=2 then the paired up point sources have a phase difference of 2pi and constructive interference occurs. Also when m=3 (or an odd number), it seems we have to revert back to two sections to explain destructive interference which gives a phase difference of 3/2pi which is destructive interference. Why does using 4 sections for m=2 give the 'right' answer of destructive interference and not 2 sections?