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Electromagnetic waves decrease in speed when propagating through a medium. The commonly accepted microscopic explanation is that photons are absorbed by electrons and then re-emitted, resulting in a decrease in the speed of light. However, my question is, if the electromagnetic wave intensity is extremely low, meaning that within a finite interaction time, such a low-intensity electromagnetic wave cannot even excite a single photon, would the electromagnetic wave still be affected by electrons (medium), leading to a decrease in the speed of light?

Wein
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    The electromagnetic wave is the photons. – Jon Custer Dec 15 '23 at 02:05
  • thanks. I mean, in quantum point of view, the photon is excitation from the EM field. Now the EM field is very low, so it needs a very long time to excite even one signal photon averagely, but the EM field does exist there together with the electron. – Wein Dec 15 '23 at 02:28
  • No, there just aren’t many photons per unit time going by. – Jon Custer Dec 15 '23 at 02:32
  • so if i focus on a specific interaction cross section, e.g. one square cm. If it requires 10 year to have one single photon passing the cross section averagely due to the very low flux density of the light, then, such weak light seems could pass the medium without any decrease of speed? – Wein Dec 15 '23 at 02:49
  • EM light is built up by photons, but photons are not electromagnetic light. Photons are modeled with quantum electrodynamics and Feynman diagrams are used to calculate the probability of an interaction happening.They always move with the speed of light c.https://physics.stackexchange.com/questions/273032/what-exactly-is-a-photon – anna v Dec 15 '23 at 05:48
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    Are photons necessary here? Propagation of waves inside medium is treated in, e.g., Chap 6 of Jackson's Classical Electrodynamics. The photons in electromagnetic media are typically not the free-space kind, but the transformed interaction picture type objects, i.e. these photons would include electromagnetic response of the medium in their formulation. Perhaps this is what question is about? BTW, absorbed and re-emitted is probably too simplistic of a picture. You will have a coupled system - free-space electromagnetic field and a lattice of atoms, and you will seek to transform it... – Cryo Dec 15 '23 at 06:28
  • ... to extract decoupled modes, in much the same way as when you have two coupled resonators you can transform the description of the system to extract two normal modes – Cryo Dec 15 '23 at 06:30
  • Absorption is probably more relevant for irreversible processes related to heating. In a loss-less dielectric medium it should be possible to describe evolution without absorption – Cryo Dec 15 '23 at 06:32
  • @annav thank you. So, why these photons propagate slower through the medium in speed less than c please? – Wein Jan 04 '24 at 05:43
  • The photons always go with speed c. It is the em radiation group velocity that slows down https://en.wikipedia.org/wiki/Slow_light – anna v Jan 04 '24 at 09:40

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