Explanation:
So this will not exactly be an answer but an explanation.
So you're studying waves I assume? If so great.
For starters we need to figure out where light is fastest; considering what you should know being that if the medium crosses an atom it slows down. This happens when they atom is absorbed. It isn't just one that slows it down it has to be multiple as it takes time to push the emission back out in the same direction.
Higher density = Slower WL
Lower density = Faster WL
Now where light and waves would travel fastest is in a pure vacuum where there is nothing to interrupt the path.
in summary any density slows it down, in a vacuum it can hit it's highest speeds.
If you still need help comment below
