Light travels through hydrogen at a speed very close to the speed of light in a vacuum, which is approximately 299,792,458 meters per second.

Here's why:

* The speed of light is primarily determined by the permittivity and permeability of the medium it travels through. Hydrogen, being a very light and tenuous gas, has a permittivity and permeability very close to that of a vacuum.

* The refractive index of a medium is a measure of how much light slows down when passing through it. The refractive index of hydrogen is very close to 1, indicating that light travels almost as fast in hydrogen as it does in a vacuum.

Therefore, while there's a tiny difference, light travels through hydrogen at practically the speed of light in a vacuum.