Single slit diffraction confusion

[adi1998]

I've found this a very confusing thing:when we locate the second dark fringes above and below the central maxima,we divide the slit into four parts and claim that pairs of waves interfere destructively as the path difference would be wavelength/2.But what if we just divided it into two parts and claimed that two rays each from one half at equal distance would always interfere constructively with path difference=wavelength?

 

[blue_leaf77]

Where did you read/hear about this from?

 

[rumborak]

Well, you uncovered the fact that the "simple subdivision of the gap" explanation is a very, very crude approximation. It gets the basic principle across that waves originating at different parts have different path lengths (and thus will interfere with each other), but of course, if you *really* want to calculate it, you have to consider each point of the gap as a wave source and add (well, integrate) the paths' interferences. That integral will actually give you the answer.