In the double-slit diffraction experiment, two interferences are observed simultaneously : diffraction interference from each single-slit, and double-slit interference (where in the double-slit interference, diffraction is ignored because we considered theoretical slits way smaller than wavelength). Intensity of the light reaching the screen behind the slits then looks like that : My questions are : 1 - why is it the diffraction part the envelope of the double slit interference, and not the other way around ? 2 - Is it actually always in this way only ? I've looked at 5 textbooks (undergrad level) and none of them justifies that. They all say something like : the diffraction act as an envelope, and the corresponding equation is the product of both equations for the single-slit diffraction and for the double-slit interference together.