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I was idly musing (as you do) and I was thinking of Fermat's principle which says that light takes the 'shortest route'. That, however, must refer to the classical situation.
Take the simplest diffraction situation of two slits - or forming the really complex diffraction pattern from a large object on a hologram plate. (Or even, now I think about it, forming an image with a lens). You suddenly turn on the illumination. The image that forms in all three cases in made up of light (photons if you insist but I'd really rather not) from many paths of different lengths.
So the diffraction pattern will not be there to start with, as only one contribution will arrive first. The path difference for a large optical system could be several (even tens of) cms, corresponding to time differences of many picoseconds. (c is about one foot per nanosecond).
So this image will form over an extended period - by no means instantaneously - as the various contributions arrive. A bit like a developing photograph.
Obvious, when you think about it, but perhaps not everyone has. Enjoy.
Take the simplest diffraction situation of two slits - or forming the really complex diffraction pattern from a large object on a hologram plate. (Or even, now I think about it, forming an image with a lens). You suddenly turn on the illumination. The image that forms in all three cases in made up of light (photons if you insist but I'd really rather not) from many paths of different lengths.
So the diffraction pattern will not be there to start with, as only one contribution will arrive first. The path difference for a large optical system could be several (even tens of) cms, corresponding to time differences of many picoseconds. (c is about one foot per nanosecond).
So this image will form over an extended period - by no means instantaneously - as the various contributions arrive. A bit like a developing photograph.
Obvious, when you think about it, but perhaps not everyone has. Enjoy.