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(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

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    $\begingroup$ I think in the original experiment, a primary single slit was used which then illuminates the double slit. Any abrupt phase change from the source would then equally affect both the slits and this phase difference remains the same or in other words phase is locked. And we obtain interference pattern. I'm sure this kind of question was asked before on this site. I hope a small search will give you more insight into this topic. $\endgroup$ – Guru Vishnu Jan 23 '20 at 14:09
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    $\begingroup$ Re, "The phase from 2s ago is in no way related to the phase from 0s ago." The light that passed through the experiment 2s ago does not interfere with the light that is passing through now. Experiments have shown that "interference" is something that happens independently for each individual particle (photon, electron, etc.) passing through the experiment. $\endgroup$ – Solomon Slow Jan 23 '20 at 14:11
  • $\begingroup$ The light needn't be coherent over a 2 second time interval. Light gets divided between the two slits and there is a path length difference (pld) between the light from each slit heading in a particular direction. As long as the pld is less than the coherence length of the light, there will be some interference. $\endgroup$ – Not_Einstein Jan 23 '20 at 18:42
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This is the original Young experiment with double slits:

ydslit

In order to get a coherent source ( lasers did not exist at that time) he made the incoherent beam of light fall on a single slit, narrow enough to become a point source in the two dimensions. By construction the light is coherent, as it can mathematically be described by spherical waves leaving the source with fixed phases. That was the way they got coherent light before lasers. The laser experiments reproduce the results.

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The answer in the link from Steve Byrnes is best. There is always some level of coherence in light, even incandescent light. The color filter and pinhole help to increase coherence ....Is coherent light required for interference in Young's double slit experiment?.

Also bear in mind that another answer in that link, points out that Dirac says photons do NOT annihilate one another, it would be violation of conservation of energy. Individually sent photons exhibit the same pattern in the YDSE, per Feynman photons have probable paths (and improbable ones) .... all based on their wave nature.

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