In a dark room when we see a spark due to static electricity, what is the nature of the spark that we’re seeing? Is it a path of ionized air? Oxygen? Would we see it if it were in a vacuum?
(BTW, as usual, this is another question that my daughter asked that I was (barely? not?) able to answer)
Ionized air, according to my high school physics teacher.
I think there would be a glow even in vacuum. Not from ionized air, but because the electrons get accelerated by the electrical field and hit the target at a high speed. There’d be lots of UV and X-ray emission as well, if the voltage is high enough. In fact, that’s how most X-ray generators work, including the one your dentist has.