I'm wondering if the speed of light in a medium other than vacuum is well defined. I explain myself: Say I am underwater and I create a laser pulse. I know that at any given time, the speed of the photons constituting the light is always c. However I also know that photons will get absorbed by the atoms of water molecules and then be re emitted in the same direction as they were. So that light will take a longer time to go through a distance than if it was traveling in vacuum. Say I have a photon detector situated at a distance of 300000 km from the laser, still under water (I know it's not possible on Earth but anyway). Some (maybe very few) photos will never meet a single atom between leaving the laser and being absorbed by the photon detector. Some other photons will be absorbed and re emitted say n times, other m times, etc. So what started as a pulse, isn't a pulse anymore when it goes to the detector. Some photons will reach the detector well before the others. Then why do we even talk about a speed of light in water? What does this really mean? Is it an approximation for small distances (in contrast with my example of 300000 km)? Is it an average of the "apparent speed" of all the photons? Or I'm all wrong and indeed, the speed of light in any medium is well defined? Edit: I just thought that maybe such a huge distance would make that almost all photons were absorbed and re emitted a very close number of times. So if it is the case, consider a very few photons going through a medium distance (say 40000 km) in a gas at a very low pressure.