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A solar furnace is a device that concentrates the sun's light on a small point to heat it up to high temperature. One can imagine that in the limit of being completely surrounded by mirrors, your entire $4\pi$ solid angle will look like the surface of the sun, at about 6000K. The target will then heat up to 6000K and start to radiate as a blackbody, reaching thermal equilibrium with the sun.

The question is: is there any way to surpass this temperature, perhaps by filtering the light to make it look like a BB spectrum at higher temp, then concentrating it back on the target?

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    $\begingroup$ That would violate the second law of thermodynamics, assuming all the optics are passive. $\endgroup$
    – Mark Eichenlaub
    Commented Jan 10, 2011 at 15:34
  • $\begingroup$ If you assume equilibrium, then there is no way. Black body must always emit as much radiation as it receives. The upper limit is given by total Sun's output corresponding to 6000K. $\endgroup$
    – Marek
    Commented Jan 10, 2011 at 17:03
  • $\begingroup$ By the way, you might want to read up on Dyson sphere. $\endgroup$
    – Marek
    Commented Jan 10, 2011 at 17:04
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    $\begingroup$ If you surround the Earth-sun system with ideal mirrors, everything on the inside will eventually heat up well beyond 6000K. Just divide the total mass by the heat derived from fusing everything to iron. $\endgroup$
    – Scott Carnahan
    Commented Jan 11, 2011 at 13:11
  • $\begingroup$ @ScottCarnahan "Ideal mirrors" is the problem. If you surrounded the sun with a Dyson sphere, with mirrors all over the inside, the fact that the mirrors are real and not ideal means that the mirrors will absorb some heat. Eventually the heat will conduct to the outside of your Dyson sphere which then emits blackbody radiation. It will come to equilibrium that is far less than the iron-fusing limit you are referring to. $\endgroup$
    – Ross Presser
    Commented Mar 16, 2023 at 21:34

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Theoretically the answer is yes. That is because the sun is not a blackbody emitter, there is an excess of UV radiation. So if you were able to achieve radiative equilibrium with only UV light (which is maybe 1% of BB radiation at those temps), you could do it. Practically, I'd think it would be just about impossible, as your filter would have to have it's innermost surface at nearly 6000K.

Note: The solar UV primarily comes from the chromosphere and corona, which is heated (in some not too well understood way) by mechanical/magnetic energy derived from convective processes. The X-ray excess is even greater than the UV excess. Even the earth gives off detectable gamma rays, and that would be impossible thermally.

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  • $\begingroup$ Can someone explain fundamentally how there could possibly be a temp limit. Just keep adding more light. $\endgroup$
    – Al Brown
    Commented Aug 23, 2021 at 6:44
  • $\begingroup$ @AlBrown From what I have been reading it's a similar principle to why you can add more and more boiling water to a container, but the container will never get hotter than the temperature of boiling water. You can add more and more sunlight, and heat a larger and larger area up to temperature, but you can't make something hotter than the temperature of whatever it is you are putting into it (when you are using passive devices like lenses). $\endgroup$
    – Malvineous
    Commented Sep 11, 2022 at 4:05
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    $\begingroup$ @AlBrown See what-if.xkcd.com/145 $\endgroup$
    – Ross Presser
    Commented Mar 16, 2023 at 21:30
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According to wikipedia, it can reach 3500-4000 °C

You can increase the temperature by choosing material which is black at visible light range and white at IR range.

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There is no limit to the degree of concentration. In theory, the entire output of the Sun could be concentrated into a small point.

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    $\begingroup$ Basic thermodynamics... a passive process like reflection cannot build a greater temperature than the thermal source. $\endgroup$
    – Muggs McGinnis
    Commented Sep 17, 2018 at 1:18

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