Take the 2-minute tour ×
Physics Stack Exchange is a question and answer site for active researchers, academics and students of physics. It's 100% free, no registration required.

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?

share|improve this question
    
That would violate the second law of thermodynamics, assuming all the optics are passive. –  Mark Eichenlaub Jan 10 '11 at 15:34
    
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. –  Marek Jan 10 '11 at 17:03
    
By the way, you might want to read up on Dyson sphere. –  Marek Jan 10 '11 at 17:04
1  
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. –  Scott Carnahan Jan 11 '11 at 13:11

1 Answer 1

up vote 2 down vote accepted

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.

share|improve this answer

Your Answer

 
discard

By posting your answer, you agree to the privacy policy and terms of service.

Not the answer you're looking for? Browse other questions tagged or ask your own question.