I thought the speed of light is a constant?
The speed of light *in a vacuum* is a constant. In simplified terms (because UNsimplified requires a discussion of quantum physics that I doubt anyone here- especially me- has the background to do), the speed of light in various materials varies between very slightly and much slower than that, based on the electromagnetic properties of that material/medium. It is this reduced speed that allows us to make prisms and lenses out of glass, for instance - the change in speed at the boundary results in a change in direction for light not hitting perpendicularly. In typical fiber, the speed of light is around 30% slower than in a vacuum.
That fiber still passes light at around 120,000 miles per second though, so traveling 3,000 miles (across the continental United States, for instance) would still occur in only 24 ms, if it were a single uninterrupted run. The majority of latency for most internet communications comes from the many nodes that the data travels through. At each node, the data is received (completely), then it is checked for errors, the next hop determined based on its final destination, and then added to a queue to be sent on that next hop. As the total traffic through a node increases, the delay for the processor at that node to process each data packet increases, which then increases the total travel time for the data, regardless of the actual physical distance traveled.
Mike