Light always moves at C ~300,000km/s.
If light were a track star running a race, it would start the race at 300,000km/s and end the race at 300,000km/s. However, when light moves through a medium other then the vacuum of space, say glass, its velocity is slowed. Let's examine this... In glass, light is still a track star running a race, but imagine this time it is running a relay race. Each atom in the glass is a teammate to which the track star must pass the baton, and each teammate must pass the baton to the next teammate in line. This baton passing takes up time, but light always travels at 300,000 km/s for every moment up to the actual baton passing, and for every moment after the actual baton passing, until the next passing of course, where it is slowed down again. Once we add up all the delays of passing the baton between several teammates, at the finish line we measure light as slowing down to about 220,000 km/s, in the specific case of glass (crown glass to be precise). In general, the denser the medium (we examined crown glass in our example), the more 'baton-passings' occur which in aggregate appear to slow down light. Diamonds, for example, are so dense a medium that light is slowed down to below 150,000km/s when inside of it due to all the atoms (teammates) that must pass the baton before the light can emerge.
In a perfect vacuum, there are no teammates thus no baton passing to slow down light so it merrily goes along its way at a constant ~300,000 km/s.
Hope this helps.
Chaos