Light Speed in Vacuum: What's the Significance?

[Nickriener]

OK, I've been wondering, what's the significance of saying "speed of light in a vaccum" if lightspeed is a constant. Is the "vacuum" part even necessary?

 

[russ_watters]

Except as a clarification that light only travels in a vacuum...light appears to travel slower through a medium but that is only because of occasional interactions between light and atoms.

 

[Nickriener]

such as your frame of reference?

 

[russ_watters]

Huh? That's not a complete sentence...

 

[Chronos]

Light travels unimpeded in a pure vacuum - where it achieves its maximum velocity. Photons traveling through a medium are absorbed then reemitted by atoms, which slows their journey. The amount of slowing depends on how long the photons are 'held' before reemitted, and the number of captures suffered before reaching an observer.