Light entering glass in outer space has a speed of c/n relative to the glass. This varying in speed within the medium is called extinction, and is completed in a very small optical path length that differs depending on mediums. For glass, it is 0.0001 mm.
Imagine that the glass moves in various uniform linear motions in outer space. The speed of light entering the glass and leaving the glass are not the same (from the perspective of the glass).
