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Many materials, such as glass, are transparent over some wavelength interval. In the photon picture, as far as I understand it, this is because there are no allowed transitions corresponding to the energy of the light for which the material is transparent. If this is so, and if these photons don't interact with the material in question, then how can light at the same time be "slowed down", i.e. propagate with a lower speed than c ?

Étienne Bézout
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    In the photon picture an elastic scattering process is described by the absorption of a photon and the emission of a photon with the same energy but changed momentum. The total average path traveled by photons between the scattering atoms becomes longer, even though the individual photon between these scattering events still travels at c. – CuriousOne Apr 29 '15 at 22:40
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    Adding on to @CuriousOne, the atom behaves similarly to a resonator. One can think that the photon dwells a while in the atom before being re-emitted. It stays in the atom for a longer interval when the energy of the photon is resonant with an atomic transition than when far away from resonance. Hence higher index (lower speed) on resonance. (Describing a photon this way, as a particle, makes me uncomfortable, but this is a case where it works sort-of ok.) – garyp Apr 29 '15 at 23:37
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    If you search around you can find a number of answers to previous questions that describe what happens in light in a medium. I want to make one important general point though: "is not absorbed" is not the same as "does not interact." Light that is off-resonance from all transitions still absolutely interacts with atoms; the light can shift atomic energy levels (this is called the AC Stark effect), and the light is correspondingly affected by the atoms. – Rococo Apr 29 '15 at 23:51
  • Alright, thank you for your comments! I'll have a look at those previous questions too. Another question however: if the refractive index is higher for resonances, why do we observe that the refractive index drops continuously with increasing wavelength rather than observing spikes at the wavelengths corresponding to resonances? – Étienne Bézout Apr 30 '15 at 00:07
  • possible duplicate of What really cause light/photons to appear slower in media? – John Rennie Apr 30 '15 at 09:26

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