It seems many posters on these forums as well as others make a distinction between “light” and a “photon” when discussing the speed of light. Most will agree that an individual proton propagates at C whether passing through air, lead, glass or a vacuum. If one defines light as a packet of photons, or a shower of photons, it allows the possibility of saying, “the speed of light is medium dependant” while knowing that an individual photon propagates at exactly C. An example of the confusion caused by the distinction can be found in refraction tables developed by persons of great authority wherein they assign to light, a velocity dependant on the properties of the medium. The title of the tables usually refer to the “speed of light”. They are either ignorant or, more likely, distinguish between light and a photon.