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Light moves slower through transparent substances because it is interacting with particles on the way through, but between those particles it's moving at the speed of light through a vacuum. You get different speeds of light because these interactions reduce the average speed of light as it moves between two points.
It's not like changing mediums slows or speeds light. Rather, changing mediums increases the average number of interactions that light is going to have to deal with on it's way through.
This is my crude understanding of this issue, and I'm sure that somebody else can improve upon or correct my explanation.
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