Technically it's any medium. The lower the refractive index, the closer the particle needs to travel to the speed of light in vacuum. But you can for example me

Technically it's any medium. The lower the refractive index, the closer the particle needs to travel to the speed of light in vacuum. But you can for example measure Cherenkov Radiation in the air (where n~=1.0003 or 99.97% of c) from highly energetic cosmic rays. https://en.wikipedia.org/wiki/Imaging_atmospheric_Cherenkov_telescope https://en.wikipedia.org/wiki/Imaging_atmospheric_Cherenkov_...