1. Sep 25, 2007

### dasher

the speed of light relative to glass, still water, or other media is not 3*10^8 m/s (otherwise known as c). This is due to the refractive index, the refraction of light itself. Is this actually a violation of the theory of special relativity?

2. Sep 25, 2007

### Demystifier

No.
Theory of relativity does NOT say that light always moves with the velocity equal to c=299792.458 km/sec. It says that IF SOMETHING moves with the velocity equal to c, THEN IT moves with that velocity for any observer.
The unfortunate fact is that c is called "the velocity of light", while such a terminology is actually misleading.

3. Sep 25, 2007

### dasher

so does this mean that the statement (or fact): "The speed of light relative to still water is 2.25*10^8 m/s." does not violate the theory of special relativity? However, can a more elaborate explanation be given to why this is so?

4. Sep 25, 2007

### Staff: Mentor

When we say "the speed of light is always c" according to SR, we always mean "the speed of light in vacuum." We're simply too lazy to write out the complete statement every single time we say it.

Light traveling through a medium does slow down, in effect. This is addressed in the Physics Forums FAQ (located in the General Physics forum):