Electric sparks tend to appear blue or purple or white in color. Why?

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    $\begingroup$ Note that the answers below also apply to lightning. $\endgroup$
    – dotancohen
    Commented May 27, 2015 at 9:51

3 Answers 3


Air is normally a bad conductor of electricity, but with enough voltage it can be converted to plasma, which is a good conductor. In a plasma, the electrons constantly bind to and leave atoms. Each time an electron binds to an atom, it emits the energy in light. As a result, the plasma glows the color of a photon with that energy. There are a few different energy levels that get involved, so the spectrum has a few different peaks. The final color depends on the gas you use. For example, neon looks red or red-orange. Air ends up looking blue, so electricity passing through air makes it glow blue.

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    $\begingroup$ +1 good answer: but do you know which atmospheric gas is mainly responsible for the color? $\endgroup$ Commented May 27, 2015 at 5:00
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    $\begingroup$ @WetSavannaAnimalakaRodVance I think you will find an answer by looking at the composition of earth's atmosphere $\endgroup$ Commented May 27, 2015 at 5:12
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    $\begingroup$ You have described recombination radiation, but this forms a continuum. What are the particular transitions in what elements that cause the colours seen? $\endgroup$
    – ProfRob
    Commented May 27, 2015 at 16:41
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    $\begingroup$ I know that in nitrogen discharges, the main emitter in the visible band is the radiative decay of N2(C3Pi) radiating downward to either N2(B3Pi) or N2(A3Sigma) (I forget which one). I don't know how these compare with oxygen emission however. $\endgroup$ Commented May 27, 2015 at 17:40
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    $\begingroup$ @RobJeffries you're right, the light emitted by a spark is a discrete spectrum, mostly from electrons which are excited to a higher energy state (but don't leave the atom or ion) when they drop back to a lower energy one. So it's not recombination radiation... Edit: ah, like you describe in your answer :-) $\endgroup$
    – craq
    Commented May 28, 2015 at 11:43

The deexcitation of nitrogen and oxygen, the primary components of air, are that of blue/purple.

See http://en.wikipedia.org/wiki/Ionized-air_glow for pictures of nitrogen and oxygen in gas discharge tubes.

  • $\begingroup$ Not only does that wiki article contain the described pictures, it contains a good answer to the question as well! $\endgroup$
    – Jason C
    Commented May 31, 2015 at 2:06

The answer is that electrical excitation of air molecules is able to produce lots of excited singly ionised nitrogen ions.

The electronic structure of singly ionised nitrogen has a number of allowed radiative transitions, where the outer excited valence electrons can rearrange themselves into lower energy configurations.

The most prominent turn out to be those transitions corresponding to emitted photons at 443, 445 and 463 nm, and it is these that are responsible for the blue airglow.

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    $\begingroup$ +1 for the first answer that mentions the relevant wavelengths. $\endgroup$ Commented May 27, 2015 at 22:14
  • $\begingroup$ Why do some air gap sparks appear orange/yellow? I've personally observed this and have asked a question about it here: physics.stackexchange.com/questions/244630/… $\endgroup$ Commented Mar 22, 2016 at 13:43

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