See e.g.: https://en.wikipedia.org/wiki/SN_1987A
See e.g.: https://en.wikipedia.org/wiki/SN_1987A
Not just that, but lights when it passes through a medium slows down depending on it's refractive index (speed of light in water is about 225 000km/h while in vacuum it's 300 000km/h). This is important distinction as recently we had an uproar when it looked like neutrinos are slightly faster than c (in the end, it was a measurement error).
You can observe effects of particles going faster than speed of light in a medium if you look at photographs of Cherenkov radiation.
How does a particle or its environment measure the particle's traversal of a truly empty space?
300 000 km per second
"Its exact value is 299,792,458 metres per second (approximately 300,000 km/s"