Is the speed of light always constant, even in non-vacuum environments?

[mjacobsca]

Often times I read in these forums that the sped of light is C in a vacuum, but is slower in non-vacuum environments. Every time I read this, I wonder if it is a misstatement. Doesn't light always travel at C, but gets interrupted by being absorbed and re-emitted countless times by intervening particles? The way I understand it, the photon travels at C, gets absorbed and/or stopped by a particle, then gets emitted, traveling at C to the next stop, emitted, C, and so on. So while light may propagate slower than C, the photons themselves never travel slower than C. Is this a correct statement?

 

[Pengwuino]

Yes, exactly.

 

[darkside00]

what about when light is affected by gravity...

 

[mjacobsca]

Are the photons being emitted the same photons that were previously absorbed? Or are they different photons with the exact same energy and frequency? And if they are different, would that imply that we don't really ever see photons from a distant source, but rather photons that were emitted just a few feet in front of me?

 

[Meir Achuz]

O The way I understand it, the photon travels at C, gets absorbed and/or stopped by a particle, then gets emitted, traveling at C to the next stop, emitted, C, and so on. So while light may propagate slower than C, the photons themselves never travel slower than C. Is this a correct statement?

No. The wavelength of a light photon is much longer than the distance between atoms, so the photon is affected by the collective action of billions of atoms. A photon in a material with index n travels at the group velocity in the material, which is less than c.

 

[A. Neumaier]

Often times I read in these forums that the sped of light is C in a vacuum, but is slower in non-vacuum environments. Every time I read this, I wonder if it is a misstatement. Doesn't light always travel at C, but gets interrupted by being absorbed and re-emitted countless times by intervening particles? The way I understand it, the photon travels at C, gets absorbed and/or stopped by a particle, then gets emitted, traveling at C to the next stop, emitted, C, and so on. So while light may propagate slower than C, the photons themselves never travel slower than C. Is this a correct statement?

This is a frequent statement but it is not true. Light in matter is describe by effective photons, not the free photons one has in vacuum, and the effective photons travel with a lower speed. See
https://www.physicsforums.com/showpost.php?p=899393&postcount=4