Short answers, pulled from the story.
Ole Rømer presented his report to the French Academy of Sciences in 1676. This presentation revealed that light takes time to cross space rather than traveling instantly.
The 17th General Conference on Weights and Measures fixed the exact number for the speed of light in 1983. This decision made the value 299,792,458 meters per second a defined constant rather than a measured quantity.
Light moves at roughly two-thirds of its maximum possible rate inside glass because glass has a refractive index around 1.5. This slowing effect occurs due to the interaction between electromagnetic waves and the material medium.
Objects with nonzero rest mass require infinite energy to reach the speed of light according to relativistic equations. Only massless particles like photons and gravitational waves always travel at this exact velocity in vacuum regardless of reference frame.
Hippolyte Fizeau developed a toothed wheel method in 1849 measuring light over one kilometer distances yielding results around 315 million meters per second. This experiment provided an early direct measurement of light speed before later improvements by Léon Foucault and Albert A. Michelson.