Light travels at the speed of light. There is no general velocity of light because velocity is a vector quantity, it also contains a direction and there is no preferred direction for light rays in...
Light changes speed when it passes through a transparent substance, based on the refractive index. For diamond, the dense crystal slows light to less than half of its velocity as measured in a vacuum.
Nothing propels it at all. It actually always goes that fast, even from its creation. To explain it to you mathematically would require the use of 4-D tensor products, so I won't bother with that...