Just to be pedantic. C is the speed of light in a vacuum. Light has different velocities when travelling though different media.
This is actualy false. The speed of light is always the same. Why does it seem slower? Because when it moves through a medium, it takes a longer route because it's reflected off of mulecules inside the medium in slight angles.
Took a little searching:
http://www.physlink.com/Education/AskExperts/ae509.cfm
Anyway, bit off topic. Just couldn't resist to correct.
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