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A neon gas tube, when passed with high voltage electric current, will emit a characteristric spectrum of color.

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This spectra lines are explained by the different combinatorial paths that electrons jumping from higher orbitals can take to reach any lower orbital states,

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The above spectral signature is unique for neon, and all other different atoms will emit different sets of spectral lines.

However, Kirchoff law of radiation due to thermal motion states that all materials, regardless of composition, shape and atomic structure, will always emit the same color spectra at the same temperature. Thus ceramic and iron will glow exactly the same color when heated to the same temperature (Do all objects at the same temperature glow the same color?).

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Suppose we now have a gas cloud of neon, which is heated to a temperature of 2500 Kelvin.

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Will the radiation color spectrum given off by neon gas at 2500 Kelvin follow the neon's specific atomic spectra signature, or the universal thermal motion radiation spectra following Kirchoff/Wein's law?

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The neon line spectrum is generated at relatively low temperatures, near room temperature in fact; its method of generation is mostly athermal.

But as you heat the neon up, the blackbody spectrum will be superimposed on the line spectrum. This will make the light from the neon tube become progressively whiter and less red/orange as the temperature goes up.

You can see this effect by comparing the light from a low-pressure sodium vapor tube (which is a brilliant, intense orange) with that from a so-called high pressure sodium vapor bulb. the tube puts out the line spectrum but the bulb can withstand higher pressures, and so you can pump more current through the ionized sodium and heat it enough to get the blackbody spectrum to kick in. High-pressure sodium bulbs are commonly used for street lighting; they are the ones that are mostly white with a noticeable tinge of orange. In the olden days, the original street lights on the Golden Gate Bridge were the lower-pressure sodium type that produced a lovely bright orange which resembled the color of the paint job on the bridge which was visible during the daytime.

You can see this same effect if you watch a large, old-fashioned mercury vapor bulb go through its cold start procedure. At first ignition, the quartz capsule with the mercury in it starts putting out a faint line spectrum with lots of blue and green lines plus all the UV ones which you can't see. Then the bulb drive circuit turns up the current and the capsule starts glowing red, then orange, then yellow and then brilliant white and the line spectrum gets overwhelmed. But it's still there, as a spectrometer will reveal.

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  • $\begingroup$ Thank you. That clarifies the question perfectly. $\endgroup$
    – James
    Nov 21, 2022 at 4:43

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