[devijvers]

No, it's correct. The light actually gets slowed down in material (anything else than a vacuum.) That has nothing to do with observation, it's an actual physical effect. The speed of light in a vacuum is constant and so is the speed of light in any particular pure material (like a pure gas.) It's just that those constant speeds are different.

[felipemnoa]

I say you are incorrect. Read this: http://physlink.com/Education/AskExperts/ae509.cfm

Here is a relevant piece:

When light enters a material, photons are absorbed by the atoms in that material, increasing the energy of the atom. The atom will then lose energy after some tiny fraction of time, emitting a photon in the process. This photon, which is identical to the first, travels at the speed of light until it is absorbed by another atom and the process repeats. The delay between the time that the atom absorbs the photon and the excited atom releases as photon causes it to appear that light is slowing down.

[RickHull]

> The delay between the time that the atom absorbs the photon and the excited atom releases as photon causes it to appear that light is slowing down.

If the photon is traveling less D over the same amount of T, I am ok with saying the velocity is lower, and it has slowed down.

[afiler]

"Well, actually, no, officer, I wasn't speeding. You see, while you clocked me at 90mph [c] between toll booths [atoms], once you factor in time at the booth, you'll see that I am actually driving much more slowly."

[eavc]

But equating decrease in velocity with "slowing down" would be confusing for most laypeople, at least.

Nobody would say that they slowed down if they increased their speed as they went through a turn.

[Jach]

"Material" is made of smaller things, which I think the GP is getting at. The actual photons that travel from electron to electron and such don't get slowed down; they effectively travel through a vacuum that is the tiny spaces inside molecules and atoms.