According to my knowledge, when clouds get charged, they produce sparks, which look like lightning to us. My question is, why does lightning have a color? It consists of electrons, and should not radiate light. If it radiates light after hitting air particles, then it should radiate only X-rays. Is that correct?

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    Lightning emits electromagnetic radiation across nearly the entire spectrum from radio waves to gamma rays. If you imagine the electrons moving to create a current (basically what occurs because this is a discharge) and the current is very abrupt, then how is this different than an antenna? You have a time-varying current, which can radiate waves. Of course I am ignoring the emission that occurs when electrons are liberated from atoms and/or molecules and all the other phenomena that lightning produces... – honeste_vivere Aug 2 '16 at 12:23
  • @honeste_vivere The reason the electromagnetic radiation has such a near-uniform distribution is because the lightning strike is a near-instantaneous signal. You could model it as a dirac delta function, and as such its frequency response covers the whole spectrum. – Devsman Aug 2 '16 at 19:40
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    @Devsman - the duration of a lightning strike is "long" compared to the characteristic time you would need (short compared to the period of the light. If light has a frequency of roughly 500 THz, the pulse duration would have to be less than 2 fs for spectral broadening. Now the mean free path of atoms in the lightning strike might result in spectral broadening... but not the "near instantaneous" nature of the strike itself (about 30 µs. It's just not that fast. – Floris Aug 2 '16 at 22:02
  • "If it radiates light after hitting air particles, then it should radiate only X-rays. Is that correct?" How can this be correct? We can see lightning! – Lightness Races in Orbit Aug 3 '16 at 11:44
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    @LightnessRacesinOrbit: That's begging the question. As Floris' answer explains, visible photons are created by another process (thermal, T=30.000K) – MSalters Aug 3 '16 at 13:48

TL;DR: Air in lightning gets hot. Hot things (like the Sun) emit light in a broad spectrum; including visible. You are right there will be emissions outside the visible - but your eye doesn't pick that up. So the flash looks blue-ish white to the human eye.


More complete answer:

The light you see is the result of the air getting very, very hot.

And just like the sun, which is very hot, is white - so is lightning.

Electrons can emit radiation in a wide range of energies, depending on the rate of deceleration. When they hit a tungsten target in an X-ray tube, they decelerate very suddenly from a very high starting point - thus Xrays. If they start with "high thermal" energies, and decelerate by hitting (lower Z, lower density) air molecules, their spectrum will be more like a black body spectrum. A hot black body - white. But there is some X-ray component in lightning as well - you just can't see it, and most of it is absorbed in the air before it reaches you.

Also - remember that if an object is very bright, with just a small fraction of its emission in the visible spectrum, it will still "look" white (or blue-ish). As an illustration, here is a calculation of the spectrum for sunlight (5700 K) and lightning (very approximately 30,000 K - obviously the temperature changes during the strike, and from one strike to another; but most of the light is generated while it is hottest, with the usual $T^4$ relationship of the Stefan-Boltzmann Law dominating). I scaled the plot for lightning to a max of 1.0 for comparison of shape, and then scaled it so you can compare its shape against the shape of sunlight. In fact, the intensity at 30,000 K is MUCH brighter than sunlight - if I used the same scale factor for all curves you would not see much...

enter image description here

You can see that lightning (assuming a temperature of 30,000 K) gives off mostly "light" in the UV - but there is a component in the visible, which will be bluish - but which our eyes perceive as white.

Putting the curves on the same log scale, you can see that the intensity is greater at the higher temperature for all wavelengths:

enter image description here

Further reading: http://phys.org/news/2008-07-scientists-source-x-rays-lightning.html

Quoting from that article:

"Nobody understands how lightning makes X-rays," said Martin Uman, a professor of electrical and computer engineering. "Despite reaching temperatures five times hotter than the surface of the sun, the temperature of lightning is still thousands of times too cold to account for the X-rays observed."

Two final thoughts:

  1. At the pressure and temperature inside the lightning bolt (instantaneous heating to 30,000 K should raise the pressure locally to 100 bar), you will get significant spectral broadening - that is, Doppler broadening (atoms moving towards or away from the observer with equal probability, at velocity around 5000 m/s), and collision broadening (short time between collisions "resets" the natural oscillations - at high temperature and pressure, the characteristic time between collisions can become short enough to matter). These things play inside the high intensity discharge lamps used in demanding lighting applications (spectrum from http://www.lamptech.co.uk/Images/Illustrations/SO%20SPD's.jpg):

enter image description here

Such spectral broadening will help "whiten" the light some more.

  1. The question of X rays was brought up in the quote above. It is quite possible that some of the high energy electrons (which after all accelerate in a field of thousands of V/m) travel considerable distance (and therefore obtain significant energy) before hitting an air molecule - their sudden deceleration can then give rise to higher energy emissions, including X rays. The presence of such accelerated electrons is not well described by the black body radiation model, and it should therefore come as no surprise that there are things "beyond the predicted spectrum".
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    Your final quote is surprising because its author seems to expect x-ray production to be thermal. A lightning bolt is a gigavolt electron accelerator with lots of sharp corners; bremsstrahlung and/or synchrotron radiation ought to follow naturally, but I don't know much about the details. – rob Aug 2 '16 at 17:51
  • @rob - I agree, it seems there are non-thermal electrons at play here, but I don't think any electron gets to the GV energy level without interacting. Perhaps though as the gas expands, "later" electrons have a longer mean free path and therefore a better chance at generating bremsstrahlung. And they "get there first" because the refractive index of air is closer to 1.000 at higher energies. Could it be? – Floris Aug 2 '16 at 18:09
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    @rob Lightning is pretty crazy, it can even create antimatter. dx.doi.org/10.1029%2F2010GL046259 – JAB Aug 2 '16 at 19:22
  • Thank you for all these answers and comments but can someone give me brief and accurate understandable summary of these answers – dr. honey Aug 3 '16 at 15:55
  • @dr.honey - here is the TL;DR: Air in lightning gets hot. Hot things (like the Sun) emit light in a broad spectrum; including visible. You are right there will be emissions outside the visible - but your eye doesn't pick that up. So the flash looks blue-ish white to the human eye. – Floris Aug 3 '16 at 16:01

The high voltage of the lightning arc separates electrons and ions, briefly forming a plasma. When the electrons and ions recombine back into a gas, the formerly free electrons drop to a lower energy state in orbit around their ions, and the energy difference is emitted as light. This is the bright flash that you see. Also, the newly recombined gas will be very hot, so it will glow briefly. The volume of hot gas is fairly small, so it can cool quickly and stop glowing.

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