Why does a light wave speed up when moved from air into hydrogen?

Answer 1

Electromagnetic waves (such as light), do not require a medium like mechanical waves (sound). Thus, matter that lie in its path actually slow it down, since the particles, rather than helping the waves propagate (as in mechanical waves), act as obstacles to the propagation of the wave. Since hydrogen has a lower density than air, there is a lower number of particles per given amount of area, thus the EM wave encounters less resistance to its propagation, thus the wave travels faster.

Answer 2

Light speed increases because hydrogen has a lower refractive index than air, allowing light to travel faster through it. Light waves bend less in hydrogen due to the reduced refractive index, resulting in an overall increase in speed.