[Fester Chugabrew]

Is the speed of light different in a vacuum than when it is not in a vacuum? Or is that a silly thing to ask?

[RadioAstronomer]

Is the speed of light different in a vacuum than when it is not in a vacuum? Or is that a silly thing to ask?

Not silly at all. Indeed the speed of light varies in different mediums with different densities. Why we see a pencil appear to bend when lowered into a glass of water.

When we talk about “c” (speed of light), we are referring to the speed of light in a vacuum.

See:

http://library.thinkquest.org/27066/lightrays/nlrefraction.html

And:

http://en.wikipedia.org/wiki/Dispersion_(optics)

And finally:

http://math.ucr.edu/home/baez/physics/Relativity/SpeedOfLight/speed_of_light.html