When light passes through glass/water/air, photons are absorbed and re-emitted by the chemical bonds, so that the speed of light in medium is reduced. However, in these media, it would appear that the atoms are distributed randomly. How is the coherence of light preserved over fairly macroscopic length scales as they pass through such medium (so that we can use say prisms in interference experiments)?

Some might talk about coarse graining -- but is there any computation that one can do to demonstrate that the randomness is indeed washed out?


There is no absorption and re-emission process when light travels in a transparent medium. Medium does absorb some portion of the light, but no re-emission happens, or re-emission is so small that it can be neglected.

  • $\begingroup$ but the refractive index >1 follows from interaction (= absorption/re-emission) with the medium. $\endgroup$ – janet hung Jul 9 '19 at 18:46
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    $\begingroup$ @janethung, refractive index has to do with polarizability. You can describe it as virtual absorption and emission using perturbation theory, but it is not real absorption/emission. $\endgroup$ – wcc Jul 10 '19 at 5:29
  • $\begingroup$ Some fluorescent substances absorb visible light and re-emit it. When this happens, after the visible light is turned off, the matter has its own light for a while. $\endgroup$ – Cang Ye Jul 10 '19 at 5:54

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