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.
[Physics] Why does lightning emit light
electricitylightningphotons
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Breakdown processes within solids are considerably more complex than those in gases. The properties of PMMA that has been injected by electrons via a particle accelerator have been studied by a number of researchers. If a PMMA specimen is bombarded by electrons that do not have sufficient energy to fully penetrate it, a layer of internal negative space charge is developed within. The depth of the layer depends on the electron beam's initial energy. During injection, a layer of positive image charges also develops on the exterior surfaces of the acrylic. Positive ions (created by the collision of the high-energy electrons with air molecules) are strongly attracted by the growing internal negative space charge layer and these ions attach themselves to the PMMA. They form a matching layer of positive space charge on all the external surfaces of the PMMA. The fields from the inner and outer space charge layers virtually cancel, leaving only a slight net external electrical field around the PMMA specimen. You can think of the specimen as being a "plateless" capacitor with opposite charges residing on the surfaces and deep inside the dielectric. When fully charged, the static electrical field between the inside space charge layer and the outer surfaces can reach 1-2 MV/cm across the intervening acrylic.
If we then penetrate the surface of a charged specimen using a sharp metal point, the E-field in the PMMA at the tip is significantly enhanced. Once the local E-field exceeds the impulse breakdown strength of the acrylic (typically 5-6 MV/cm), avalanche breakdown begins, and the excess charges in the negative space charge layer begin flowing through the newly-created streamer channels that originate from the metal point. Since the discharges actually originate from a more positive point and propagate into a more negative region, the resulting Lichtenberg figure is a highly-branched positive Lichtenberg figure. The metal point can be grounded (for safety reasons when discharging large specimens) or left floating for small specimens. In either case dielectric breakdown will occur with no external voltage source being necessary, since we have well over a million volts of potential available between the internal charge layer and external surfaces of the specimen itself.
The actual breakdown mechanisms occur extremely quickly. Members of our group have measured discharge propagation through PMMA at between 8.5e5 to 1.3e6 m/s using both electrical and optical techniques. Through field enhancement, the E-field at streamer tips may exceed 20 MV/cm - sufficient to cause dielectric breakdown and conduction through a variety of avalanche and field emission mechanisms. Energy to drive streamers further into virgin material is provided by the stored electrostatic energy residing within the space charge of undischarged material ahead of advancing streamers, and the process continues, almost explosively, until (most of) the space charge energy has been depleted. This energy can be quite significant. For example, the electrostatic energy associated with the space charge within a fully-charged one foot square by one inch thick piece of acrylic can be almost 1 kJ!
An excellent discussion of the breakdown mechanisms of solids going down to the molecular level can be found in the following PhD thesis (and many other subsequent papers) by P. P. Budenstein: Effects of excess charge density on dielectric breakdown in solids" at http://www.dtic.mil/cgi-bin/GetTRDoc?AD=ADA048267
A layman's description about how we create 3D acrylic Lichtenberg Figures and some theories regarding their formation can also be found on our web site at: http://www.capturedlightning.com/frames/lichtenbergs.html
I know this is a little more than you asked for, but lightning is very interesting.
A lightning event is usually called a flash and lasts about 0.5 seconds. It consists of a near-invisible stepped leader followed by a very bright return stroke backwards along the path of the stepped leader. Following the first stroke, there may be additional strokes in the flash, following nearly the same path of the first one. There may be slight deviations due to other dim leaders called darts.
Over half of all lightning flashes happen within a cloud and are called IC discharges. The type of flash of most practical importance is the cloud-to-ground (CG) lightning. Other rarer types of lightning are cloud-to-cloud and cloud-to-air flashes. Note that the bright flash that we see is the return stroke, so cloud-to-ground lightning will appear to start near the ground and zoom upward; the initial stepped leader happened first, starting in the cloud.
The typical charge separation in a cumulonimbus cloud results in the top of the cloud having a net positive charge, near the bottom having a net negative charge, and occasionally the extreme bottom edge having a small positive charge. The overall effect is that a large negative charge (of magnitude 15 coulombs) is closer to the ground than the positive charge. This charge structure causes the ground to become positively polarized as negative electrons are ``pushed'' away by the cloud.
Air is normally an insulator, but if the charge separation per distance is too large (either because of large charge or small distance), it can become a conductor. The initiation event is unknown, but scientists currently speculate that either atmospheric radioactivity (from Radon-222) or creation of ions from stratospheric reactions with cosmic rays (solar protons, other charged particles, or high energy photons) can trigger a flash.
In any event, negative charge moves in steps about 50 meters long through a small conductive pathway in the air, pausing from 20 to 50 microseconds at each step, reaching the ground in a few hundredths of a second. The current in this leaders is between 100 and 1000 amperes. After each step, the leader shifts direction as a new conducting path opens.
After the leader reaches the ground, a tremendous burst of energy is released in the return stroke along the path of the leader as the positive ions in the ground and electrons in the leader combine. This recombination produces a current of 20000 to 30000 amperes, and a temperature as high as 30000 K. The temperature of the surface of the Sun is 5800 K. This tremendous release is accompanied by a bright optical flash, an electromagnetic pulse, and the rapid expansion of heated air. The typical size of the return stroke channel is 1-2 inches in diameter.
If the initial stroke doesn't resolve the charge separation, more charge from the cloud will travel to the ground along the original path. This is called a dart leader. When it reaches the ground (about 10 times faster than the stepped leader), an additional return stroke occurs along the path. The time between strokes is on the order of hundredths of seconds but could be as long as a tenth of a second. Anyone watching a lightning storm has seen multiple strokes along the same pathway.
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A very interesting question, especially because of the discussion that it spawned.
All the answers here seem to revolve about two different mechanisms:
It's not very easy to find authorative sources on either, but googling for "spectrum of lightning" turned up some sources of interest:
Nature 6, 220 (18 July 1872) | doi:10.1038/006220b0:
this paper
and a bunch of others.
So, in conclusion, why lightning emits light is because:
The surrounding air gets superheated, which will emit
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).