[dholowiski]

Don't forget... c is the speed of light 'in a vacuum'. Light travels slower through gas, water or glass. Light has even been slowed down to walking speed in a laboratory. The speed of light is not constant. The speed of light in a vacuum is. We assume.

[DougBTX]

> Light travels slower through gas, water or glass.

There is a good description of what is going on in this Stack Exchange post:

http://physics.stackexchange.com/questions/13738/propagation...

It explains why saying "c is the speed of light" makes sense, because when we say light is traveling more "slowly" through a material, we are including the time spent interacting with the material, being absorbed and re-emitted.

I'm bristling a little at your statement that "the speed of light is not constant". Imagine two men walking at the same speed from A to B. But one of them is walking in a straight line, while the other is zig zagging. It would be fair to say that the one walking in a straight line is travelling from A to B faster, even though they are both moving at the same speed. The speed of light is a constant, it is just that light travelling through a medium doesn't necessarily spend all of its time travelling in one direction.

[felipemnoa]

>>The speed of light is not constant

I think this is wrong. Regardless of the medium the speed of light is always constant. It seems to slow down because the photons are getting absorbed and re-transmited by atoms. But the speed of light is always the same regardless.

[dholowiski]

After much research... the concept is correct, although 'absorbed' and 'retransmitted' are not the right words to use. http://en.wikipedia.org/wiki/Slow_light

"Light traveling within a medium is no longer a disturbance solely of the electromagnetic field, but rather a disturbance of the field and the positions and velocities of the charged particles (electrons) within the material. The motion of the electrons is determined by the field (due to the Lorentz force) but the field is determined by the positions and velocities of the electrons (due to Gauss' law and Ampere's law). The behavior of a disturbance of this combined electromagnetic-charge density field (i.e. light) is still determined by Maxwell's equations, but the solutions are complicated due to the intimate link between the medium and the field. Understanding the behavior of light in a material is simplified by limiting the types of disturbances studied to sinusoidal functions of time. For these types of disturbances Maxwell's equations transform into algebraic equations and are easily solved. These special disturbances propagate through a material at a speed slower than c called the phase velocity."

As another commented pointed out, you're splitting hairs. The "speed" of light and how fast it is "moving" depends on how you define those terms. The second paragraph of the wikipedia speed of light article has it right "The speed at which light propagates through transparent materials" - which does change.

[Confusion]

That's splitting hairs in a way that makes you deviate from standard usage of the term. Physicists say things like "the speed of light in water is lower than the speed of light in a vacuum".

[felipemnoa]

But to a lay person that statement does not mean the same thing as for a physicists. To a lay person it sounds as if literally the photons slow down. And I bet that a lot of people repeat this statement thinking that light actually slows down.

[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.