Thank you very much for taking the time to respond to my query. I'm trying to understand a graphic such as the one at http://hyperphysics.phy-astr.gsu.edu/hbase/phyopt/dslit.html. I'm guessing that the single slit envelope shows the pattern you'd get if you closed one of the slits and doubled...
Explanations of 2-slit interference sometimes show the interference pattern compared to a 1-slit diffraction pattern, and state that the latter represents a limit or envelope for the former. Is it correct to interpret the patterns as representing (directly or indirectly) the number of photons...