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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 May 27 '15 at 9:51
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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? $\endgroup$ – WetSavannaAnimal May 27 '15 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 $\endgroup$ – Xeren Narcy May 27 '15 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? $\endgroup$ – Rob Jeffries May 27 '15 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. $\endgroup$ – Godric Seer May 27 '15 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 May 28 '15 at 11:43
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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.

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  • $\begingroup$ Not only does that wiki article contain the described pictures, it contains a good answer to the question as well! $\endgroup$ – Jason C May 31 '15 at 2:06
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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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protected by Qmechanic May 28 '15 at 14:31

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