Question

Why does light travel faster in a vacuum?

Answer 1

because their pressure is low.

Answer 2

Because, vacuum is optically least dense. understand it like this, your speed of propagation will be less in a path full of traffic (optically dense) and will be high when traffic is low (optically rarer).

Answer 3

in a medium with refractive index n>1, the light speed is:
\[\Large v = \frac{c}{n}\]
and c is the light speed in vacuum. Now being n>1, we see that:
\[\Large v = \frac{c}{n} < c\]