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I have always heard that the inconsistency in explaining atomic models with classical mechanics was that the study of electrical charges had shown that whenever a charge is accelerated, it emits light (and thus there is no way to sustain circular movement of electrons around a nucleus).

Since then it seemed interesting to me the possibility that perhaps a very natural/commonplace phenomenon in which we can see light emitted from an accelerated charge could be lightning. But then it also occurred to me that perhaps the light of lightning is caused by the chemical reactions that happen in the path of the ray (oxygen is converted to ozone if I am not mistaken, there may be others).

I did try to search the internet a bit, but at simple glance at least the answer seemed not to be obvious. Does anybody here know which case is it? what other common or interesting cases of light emission due to charge acceleration are there?

NOTE: I am tagging this as quantum mechanics because, like I said before, the most likely place I think this information might come up other than when studying electrodynamics is as a motivation on the development of the quantum theory. Please mods feel free to modify it if you disagree or think other tags should apply and apologies for any inconvenience.

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    $\begingroup$ I think that most of lightning's emission is just plain thermal. It gets pretty hot. Free electron laser would be something to look at. And synchrotrons. And x-ray tubes (not characteristic radiation, but Bremsstrahlung). I'm not sure that any phenomenon occurring naturally shows this. It's the middle of the night here, and I can't sleep, so maybe there is something obvious that slips my mind right now. $\endgroup$ – Jarosław Komar Mar 9 '15 at 3:13
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The sort answer is no. Although the pulse of electric current will surely emit in the radio band as a normal antenna would via the process you're describing, emitting in the visible spectrum using the same process would require extreme acceleration of the electrons. This essentially the reason why you don't see visible light antennas. Most visible light is actually produced by electronic transitions in atoms or molecules.

In the case of lightning, the current through the air is possible because the air is partly ionized by the electric field. This ionization is then maintained and maximized by the heat generated when the large current of a lightning bolt flows through the ionized channel. This heat is what produces most of the light. The heavily ionized air, now in a state of plasma, has many air molecules in an excited electronic state. When these excited states relax to their normal state, they emit light, much of it in the visible spectrum.

On another note, you are partly right when considering x-ray production in lightning. These can be produced by energetic electronic transitions, which in air would not be possible with light particles, or by Bremsstrahlung radiations, which is essentially acceleration/deceleration of electrons.

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  • $\begingroup$ Thanks! This is what I was looking for. One question, though: I don't understand why you first mention that accelerated electrons normally emit in radio (which has less energy than visible) and would require extreme acceleration to emit in the visible spectrum, but at the end you say that in the case of lightning this may be the reason for the Xray emission (which is more energetic than visible). $\endgroup$ – Nordico Mar 20 '15 at 14:19
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    $\begingroup$ The accelerations required would be extreme on a macro scale with experimental apparatus. On the scale of the nucleus of an atom, the acceleration that a fast electron being deviated by a nucleus can be sufficient. $\endgroup$ – G. Bergeron Mar 20 '15 at 21:46
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"Most cloud-to-ground lightning strikes come from the negatively charged bottom of the cloud traveling to the positively charged ground below. Cloud-to-ground lightning bolts strike the tall objects, like trees and buildings."

www.weatherwizkids.com/weather-lightning.htm is not a prestigious source, but for these purposes, it is good enough.

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  • $\begingroup$ Ok, I do know the generals of why lightning is produced (the whole accumulation of charges thing), but I don't think the page explains the direct source of light emitted (at least I didn't found it). $\endgroup$ – Nordico Mar 9 '15 at 14:05

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