(Might seem like a duplicate but is not)
The original Young's double slit experiment makes use of a single incoherent source being incident on two different slits to get two coherent sources.
The original source is incoherent.i.e There is no constant phase relation in the light being emitted.So The phase from 2s ago is in no way related to the phase from 0s ago.
But in YDSE.The two waves that interfere were not produced at the same time. So they are not exactly coherent.Also the two slits are not exactly equidistant from the source.Even at 1-2 centimeters pattern is still observed
How can these two beams interfere?
(It isnt due each wave (emitted by an atom )extending for some time ..So that for small distance difference the phase is still constant,is it)(If it is not..Then could you make sure that your answer clearly explain the fact 1 source with "Time LAg" would do but 2 lasers wouldn't) do