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What exactly is causing the electric discharge coming from the clouds to emit light while traveling through the air. I've read and thought about it a little but with my current knowledge I cant really figure what is happening at a subatomic level. In other words, what causes the emission of photons during lightning. What I want to know is, is there some sort of collisions taking place that produces the photons we see, or is it some other interaction that is causing it?. I don't know too much about particle physics, I've only read a couple of pages in the Introduction to Elementary Particles by Griffihts. So if you could keep the answer simple then that would be great.

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Because it's hot. –  zhermes Jul 15 '13 at 20:50
    
@zhermes Yeah, but that's not really the answer that I'm looking for... –  Reds Jul 15 '13 at 20:56
    
Why not? (I mean, if you clarify your question to explain why something like that wouldn't be a sufficient answer, you'll probably get better answers. Not that there's anything really wrong with your question as is.) –  David Z Jul 15 '13 at 21:01
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Well, I'm glad you ended up selecting the answer which says, because it's hot. :P –  zhermes Jul 16 '13 at 3:38
    
May I ask why does the electric current in a tungsten lamp make it glow? –  Ali Jul 16 '13 at 15:03

5 Answers 5

Why does it radiate? You want an explanation on a sub-atomic level? Well then, rather than look to the fact that lightning makes the air 'hot' (of course it does), what you really need to know is this: whenever an atom changes energy level (which outside of nuclear reactions means an electron jumps from one orbital to another at a different energy level), it either absorbs a photon (if going to a higher level) or gives one up (if going to a lower). Simply changing kinetic energy (of the whole atom) does not allow all the conservation equations to add up (yes, I am skipping details here).

Now to be sure, lightning generates such a high voltage that it starts by ripping electrons away from atoms completely, which based on the above principle you would expect means absorbing photons, not releasing them. And in fact, it is by absorbing photons from the electrical field generated by lightning that the ionization takes place (because at the most fundamental level, all interactions between light and matter are exchanges of photons). But then the ions collide with other ions and neutral atoms, losing energy and therefore giving up photons.

All this happens very fast, so we see the photons given up in this last phase immediately. There are other processes going on, but they all have in common the principle I mentioned above: transitions from one energy level to another are accompanied by emission/absorption of photons.

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+1 I agree here. Light emission has nothing to do with temperature increase. –  Andreas H. Jul 16 '13 at 2:12
    
You are affirming the consequent here; if an electron changes energy state in the electron, a photon is absorbed/emitted (true). If a photon is emitted, en electron must have changed energy state (false). That second one is false, because there are more mechanisms by which photons can be emitted (electrons will also emit photons when vibrated, accelerated, etc.) Otherwise, how would you explain the continuous spectrum of thermal radiation? –  Rody Oldenhuis Jul 16 '13 at 9:32
    
@RodyOldenhuis: *black body radiation –  Rody Oldenhuis Jul 16 '13 at 15:24
    
@Rody I am not "affirming the consequent" here -- or anywhere. When "electrons are vibrated, accelerated" etc. they change energy levels, if they emit photons. This was the reason quantum mechanics was discovered: 19th century physicists realized that electrons in circular/elliptical orbits around the nucleus were being accelerated, yet were not radiating as Maxwell's equations insisted they must. The paradox was finally resolved by giving up on describing electron trajectories, describing radiation emission/absorption in terms of changing energy levels, NOT in terms of accelerating charges. –  Matt J. Jul 16 '13 at 23:39
    
Ok, then how do you explain something like synchrotron radiation? –  Rody Oldenhuis Jul 17 '13 at 6:34

A very interesting question, especially because of the discussion that it spawned.

All the answers here seem to revolve about two different mechanisms:

  1. thermal radiation of the 50.000K plasma
  2. radiation due to recombination of the resulting plasma

It's not very easy to find authorative sources on either, but googling for "spectrum of lightning" turned up some sources of interest:

  • this website.
  • Nature 6, 220 (18 July 1872) | doi:10.1038/006220b0:

    But besides this line spectrum I repeatedly saw a continuous spectrum with bright bands, which might have been the low temperature nitrogen spectrum, though I feel no certainty that such was the case. There seems, however, no doubt that lightning gives two different spectra, one of bright lines, and the other continuous;] unless indeed the latter be identical with the former, but with the lines much expanded.

  • this paper

and a bunch of others.

So, in conclusion, why lightning emits light is because:

  1. The surrounding air gets superheated, which will emit

  2. Recombination of that plasma (emission lines at $N_2$ and $O_2$ locations)

Since a "single" lightning strike normally consists of several strokes, these processes can be repeated several times in rapid succession, with additional side effects (for example, small pockets of glowing hot air at reduced temperatures may break off during or after strokes, which also contribute to the radiation of light).

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Interesting answer. I agree that when the air is do hot, it also should emit thermal radiation. If it would only be electric field excited then the lightning should probably be much shorter, like in a flash lamp? So there should also be thermal re-excitation of the molecules atoms. –  Andreas H. Jul 16 '13 at 14:35
    
What I dont understand is how this can be a continuous spectrum. For gases even thermal radiation should be a line spectrum, shouldn't it? Maybe in 1872 spectroscopic resolution was not so advanced as today and it was really a line spectrum blended together that he saw (Compare also the Oxygen A band, which also appeared to be a "line" in the first place). Have you found a reference where the 50000K are coming from (must be a spectroscopic observation, I guess) –  Andreas H. Jul 16 '13 at 14:35
    
@AndreasH.: I'll say immediately, I'm no expert :) But: I guess that during recombination, there may be electrons and ions with all sorts of energies flying around. Recombination of a random ion and electron would then result in a photon with essentially random wavelength, within the limits described by black body theory. –  Rody Oldenhuis Jul 16 '13 at 15:29
    
+1 for good googeling and references. –  Asphir Dom Jul 17 '13 at 9:36

The emission of photons is caused by acceleration of charged particles (air in the lightning becomes partially ionized - a mixture between ions and electrons = plasma) which is called thermal radiation.

You can read more about it here: http://en.wikipedia.org/wiki/Thermal_radiation

Basically the air glows because of its high temperature.

There is also a fraction of emission lines (light caused by excited electrons in atoms)

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Is it Thermal radiation or Black body radiation or both? I am asking since @Olin Lathrop said it was black body radiation that you see when the lightning strikes. –  Reds Jul 15 '13 at 22:13
    
Black body radiation is just a special (ideal) case of a thermal radiation which tells you what is the form of the spectrum of this radiation (how many photons of each energy you see). Process itself is called thermal radiation. –  Asphir Dom Jul 15 '13 at 22:23
    
Is it really thermal radiation? I thought the excitation of molecules is caused by the electric field (where the de-excitation causes the light emission) and not thermal motion. The temperature increase is also occuring, but after the light emission and not as its cause. Am I correct here? –  Andreas H. Jul 16 '13 at 2:05
    
Emission lines are also present. Nevertheless main effect is thermal radiation. Same is true for sun corona :) –  Asphir Dom Jul 17 '13 at 9:30

Well I think a lightning is a gas discharge and the light we see is the spontaneous emission from the exited gas molecules.

They are excited because of the ionization (which is a consequence of the high electric field) that makes possible current flow. When they relax in their ground state, light is emitted. Same principle as in fluorescent tubes, but the current is much higher. While the former is a glow discharge, a lightning is a short arc discharge which has strongly different electrical characteristic and has much higher temperatures (http://en.wikipedia.org/wiki/Electric_discharge_in_gases) (thanks to Georg here).

I think to speak of blackbody radiation is not correct here, because a gas with its resonant emission-absorption character is not a blackbody (although blackbody radiation is spontaneous emission, but not vice versa).

I would also hesitate calling it thermal radiation because the light emission is caused by the ionization due to an electric field. An at this very moment the gas is not in a thermal equilibrium (which is the reason why it can emit light at all!). The point is that the molecule is excited because of electric field and not because of thermal motion (as it needs to be for thermal radiation).

UPDATE: Apparently, since it is a arc discharge type, also thermal re-exitation of the molecules/atoms takes place, which then does, in addition, also generate thermal radiation (not to confuse with blackbody radiation, because this thermal radiation also has a line spectrum)

It would be interesting to to relate the time for the arc to reach thermal equilibrium to its total duration, to quantify the relation of thermal to non-thermal light emission. But this is unclear to me.

I think that lightning is mostly caused by emission from nitrogen (~70 % of air) because of the blueish-violet color. Nitrogen has emission lines there, try googling a nitrogen laser which has just this color.

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I don't know whether or not this is right, but I do know that none of the other answers have demonstrated that thermal emission is the dominant cause, so +1 for bringing up this possibility. –  Chris White Jul 16 '13 at 5:55
    
A lightning discharge is not a "gas discharge" in that classical sense. Lightning discharge is a current of some thousand amperes for milliseconds, and under such conditions you get thermal radiation. (of course not black body, because air is not black :=) This kind of discharge is called a sparc, if it would last longer, it would be called an arc. –  Georg Jul 16 '13 at 9:39
    
@Georg: thanks, I Updated my answer. –  Andreas H. Jul 16 '13 at 14:43

The massive flow of electrical current occurring during the stroke rapidly superheats the completed leader channel, forming a highly electrically-conductive plasma channel. The core temperature of the plasma during the stroke may exceed $50~000\text{ K}$, causing it to brilliantly radiate with a blue-white color. Once the electrical current stops flowing, the channel cools and dissipates over 10's or hundreds of milliseconds, often disappearing as fragmented patches of glowing gas. The nearly instantaneous heating during the stroke causes the air to explosively expand, producing a powerful shock wave that is heard as thunder.

[Adapted from Wikipedia ]

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I thought I remember reading somewhere that thunder is actually produced when the plasma cools and the air collapses back into the nearly empty space. However, I don't have a reference for that. –  Olin Lathrop Jul 15 '13 at 21:30
    
@OlinLathrop Then how do you explain that a near observer can see the lightning and hear the thunder "simultaneously"? –  metacompactness Jul 15 '13 at 21:32
    
Both the initial expansion and the collapse happen close enough to instantaneously in human time. Also, you'd have to be very close to not have the sound propagation to your location dominate. Just 30 meters is 100 ms. Anyone that close to a lightning strike has other things on their mind, and the few times I've been that close, I'm not so sure that the flash didn't seem to happen a little before the clap. Could be propagation delay though. –  Olin Lathrop Jul 15 '13 at 21:39
    
@metacompactness At that temperature, most thermal emission will be well into the UV. Do you know if this has been measured? (not doubting here - I'm just looking for more references) –  Chris White Jul 15 '13 at 22:07
    
I agree with the explanation of thunder but not with that of lightning. Is temperature really the cause for the light emission? I am not so sure. Temperature increase should be a secondary effect not directly related to light emission. –  Andreas H. Jul 16 '13 at 2:10

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