31
xkcd #3238: Soniferous Aether
(imgs.xkcd.com)
Title text:
Imagine you could ride alongside a sound wave. It would probably be pretty cool, right? We're putting in a departmental budget request to buy a really fast plane so we can check it out.
Transcript:
Transcript will show once it’s been added to explainxkcd.com
Source: https://xkcd.com/3238/
iirc light does move through the fiber itself at more or less 1c its just that it doesn't take straight path due to all the internal reflections which is what causes the slow down.
c is speed of light in a vacuum. Light does go slower in other media. This is why refraction, among other things, exists. I'm not sure what the speed of light in fiber is, and it may be very close to c, but it will not be c.