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On YouTube I have seen someone build a RF emitter (probably in the 10 MHz range with about 5000 Watts) and then use it to ignite magnesium strips.

Magnesium usually burns with a bright white flame in air. However in the RF field it burns with different colors switching been red, green and orange in a random fashion. My source is this video at 15:20.

I know about the Zeeman effect, which shifts spectral lines somewhat around. I have not done the calculation, but I cannot imagine this device to generate the required field strength of probably some 10s of Teslas to visibly change the spectrum.

So, how can this effect be explained?

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  • $\begingroup$ From viewing the video, this seems as likely to be chromatic aberration (or some other optical or transduction imperfection of the recording mechanism) as anything else. Because the sensor is super-saturated at the central region of the arc, I would trust what's seen on the edge of it. Or am I missing something here? $\endgroup$
    – tom10
    Commented Feb 13, 2022 at 17:19
  • $\begingroup$ If its a effect in the recording due to the brightness, why does it not happen in the beginning, where he burns the magnesium without any voltage applied? Also in the end, where the voltage is switched off you can see the magnesium burn with a steady white flame. And giving the experimenter is not lying, he is also commenting on how it produces all these colors. You can also see the whole set lighting up in these different colors, not just the edge of the arc. So, I think I would doubt that. $\endgroup$
    – iblue
    Commented Feb 13, 2022 at 18:23
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    $\begingroup$ It'd probably be worth ruling out simpler explanations like impurities in the magnesium that are contributing to the color. After that, maybe the color is changing with temperature for some reason? The Zeeman effect should be negligible (magnetic fields won't be strong enough to change transition energies by that much). Same goes for the Stark effect (electric fields). $\endgroup$
    – user34722
    Commented Feb 13, 2022 at 20:25
  • $\begingroup$ When I look at the Wikipedia article, there is a strong stark effect in hydrogen at only 5000 V/m, while the field in this experiment should exceed that by far. There is also an AC stark effect. So, this is may actually be the effect. $\endgroup$
    – iblue
    Commented Feb 14, 2022 at 6:43

1 Answer 1

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The OP requires: "No speculation please". I would say, this is a tall order. As there are no other answers so far, I would like to try to answer, but I cannot promise I will be able to do without speculations, as we don't have much information.

Let me start with the following note. The discharge area remains white, and it is the periphery of the discharge that acquires different colors. To demonstrate that, I made a couple of screenshots from the video, cropped them, and then replaced white pixels with black ones (in Mathematica).

Cropped image with red periphery:

enter image description here

Same image with replaced white pixels:

enter image description here

Cropped image with green periphery:

enter image description here

Same image with replaced white pixels:

enter image description here

So what is the mechanism through which the discharge affects its periphery, sometimes at a significant distance from it? I suspect that it is photoionization. Electrons in the discharge can acquire significant kinetic energy in the electric field, generate bremsstrahlung (with energy of dozens eV or higher) in collisions with atoms, the bremsstrahlung photoionizes atoms (within the discharge, but also on its periphery), and then electron transitions in the ionized atoms (on the periphery, it is mostly oxygen and nitrogen atoms) cause radiation at characteristic atomic frequencies (see, e.g. [1] and [2]). Within the discharge itself all colors are saturated, resulting in white color.

Let me try to answer a couple of questions.

Why does not burning magnesium color the periphery? Because electrons do not acquire sufficient energy without external electromagnetic field (the temperature of magnesium flame is about 2500K, which corresponds to something less than 0.3eV).

Why do the colors on the periphery change? I don't have a clear idea, but let me note that there is obvious instability of the discharge in the video, which probably can cause some relatively slow color changes.

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    $\begingroup$ (+1) What you posit seems quite plausible: basically, an aurora in a room. $\endgroup$
    – Ed V
    Commented Apr 9, 2022 at 12:13

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