How fast does light travel through hydrogen?

Answer 1

Light travels at c which is 299,792,458 meters per second. In a transparent medium light is absorbed and re-emitted by the atoms or molecules. This causes a time delay and means the apparent speed seems less than c. While travelling between each atom/molecule light still travels at c.
The relationship between c and the speed of light in the medium is given by the simple relation v = c/n, where v = velocity, c speed of light and n is the refractive index.

For H2, hydrogen gas the refractive index is 1.000132, therefore the speed of light in hydrogen gas is 299,792,458/1.000132 = 299,752,891 m/s.