# Why do we say “the speed of light in vacuum” if it is always constant? [duplicate]

Why do we say "the speed of light in vacuum" if it is always constant, in all mediums?

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–  Wouter Oct 3 at 19:13

## marked as duplicate by Emilio Pisanty, Qmechanic♦Oct 3 at 22:54

The speed of light is always the same in vacuum, the only thing that differs is that in a medium, it goes at $c$ from an atom to another, so it travels more distance and we have the impression when it comes back that it had a smaller velocity, hence, smaller speed. –  Adobe Oct 3 at 17:53