I know this question has been asked many time but my question is bit different here. What i mean by the above question is that I recently got to learn that light actually does not exclusively take the path taking least time. Light, infact, also takes nearby path other than that taking the least time but the reason why we see light taking least time is that the other path interfere with one another and cancel each other out. Now my question is that exactly how does this interference and cancellation occur? Also why does this interference and cancellation also not occur with path taking the least time? I am not very advanced in math so please if anyone can try to theoretically explain this or try to go easy on the math, I woud greatly appreciate it. Thanks.

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    $\begingroup$ I think that if we envision light as being photons, the least is actually the most probable path. $\endgroup$ Apr 8, 2017 at 15:19
  • $\begingroup$ Photons actually do take every path. Turn a light on and then go stand at different points of view. You'll see that the path of least time is not exclusive, it is everywhere. It's easier to derive this with individual photons instead of waves. $\endgroup$ Apr 8, 2017 at 15:55
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    $\begingroup$ TLo is asking how the waves that take the paths that are not least time interfere with each other to cancel out. It is a clear and standard question. It has nothing to do with seeing light spread uniformly (where you see scattering), not with light as particles. Light is both particles and waves, the truth is in the electromagnetic theory classically and QED quantum mechanically. The explanation is best done with a simplified exposition of how Feynman path integral formulation works, or the Huyigings (spelling?) principle classically and how the different wavelets cancel out $\endgroup$
    – Bob Bee
    Apr 8, 2017 at 17:37
  • $\begingroup$ Related: physics.stackexchange.com/questions/375170/… $\endgroup$ Aug 10, 2018 at 8:03

1 Answer 1


Fermat's principle is one expression of it, you can see it derived at https://en.m.wikipedia.org/wiki/Fermat%27s_principle

It is a consequence of Huygens's principle, which is explained and where you see the wavelets cancelling out in the article atwikipedia at https://en.m.wikipedia.org/wiki/Huygens–Fresnel_principle. It is also derived there. It shows that rectilinear or plane wave propagation (i.e., the simplest mathematical way to figure it out from A to B, taking A and B at infinity) is also explained that way. Fresnel showed that in 1816.

The Feynman QM formulation is in his Lectures in Physics, you can google more. It has also been generalized in field and quantum field theory to the principle of least action, actually for any field, not just light. See for instance https://en.m.wikipedia.org/wiki/Principle_of_least_action


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