Question

does speed of light vary in different mediums?

Answer 1

Yes it does,

Different mediums have different OPTICAL densities. Some are optically denser , in which the light travels at a slower speed and bends towards the normal, whereas other materials can be optically rarer in which the light speeds up and bends away from the normal

NOTE- OPTICAL density has nothing to do with the molecular density or just density as we call it. It is totally different property. A material may have high optical density than other but it may be less dense than that other object.