Why use a single slit before double-slit diffraction?

[binbagsss]

Erm, why do you use a single slit initially? - to produce 2 coherent non-identical sources, but I don't understand why you can't just diffract the beam through the double-slits initially, I thought this would produce 2 coherent sources.

Thanks in advance... =]

 

[nonequilibrium]

Okay imagine you only have two slits and to the left of them is your source, but your source is a point source that is at the height of the first slit. Now, when the waves hit the bottom slit, it'll probably have a phase shift with the top slit, since they haven't traveled the same distance. They will retain this phase shift. You'll still get an interference pattern, but you'd have to keep record of the phase shift and we don't want that. The extra slit in the beginning, that is placed symmetrically, resolves this. Personally I just prefer taking two slits and saying "the incident waves are parallel plane waves striking parallel with the wall" :) (of monochromatic light)