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Can you use a magnifying glass and moonlight to light a fire? (2016)

what-if.xkcd.com

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Re: Can you use a magnifying glass and moonlight to light a fire? (2016)

#31

Post by landryraccoon »

robertelder wrote: post #11

I'm a bit skeptical of the conclusion and the reasoning process used to arrive there. For example, the article states "In other words, all a lens system can do is make every line of sight end on the surface of a light source, which is equivalent to making the light source surround the target." If you forget about optics for a second, imagine that the outer surface of the sun were wrapped around a point (think of the…

That doesn't work because when the penny is hotter than the sun it radiates an equal amount of energy back to the sun.

The sun is a blackbody, it radiates light because it's hot. Once the penny is the same temperature it will radiate back at the sun until the two are in equilibrium.

Re: Can you use a magnifying glass and moonlight to light a fire? (2016)

#32

Post by shkkmo »

robertelder wrote: post #11

I'm a bit skeptical of the conclusion and the reasoning process used to arrive there. For example, the article states "In other words, all a lens system can do is make every line of sight end on the surface of a light source, which is equivalent to making the light source surround the target." If you forget about optics for a second, imagine that the outer surface of the sun were wrapped around a point (think of the…

I think the answer here is that you can't do this. I don't think you can build such a system to redirect all the light from the sun to a surface with a smaller area than the sun because of "conservation of étendue".

Re: Can you use a magnifying glass and moonlight to light a fire? (2016)

#33

Post by landryraccoon »

jplee wrote: post #29
jstanley wrote: post #9

Earlier quoted context omitted.

Right, but that point would be exactly as hot as the surface that the light was coming from, and no hotter. (Is the argument as I understand it).

If this is true, how do laser cutters work? The focused beam can melt through steel, but surely the temperature of the gain medium is much lower than the melting point of steel? But the thermodynamics argument seems like it makes sense too...

Lasers have negative temperature. The light emitted from the sun is nothing like laser light.

https://en.wikipedia.org/wiki/Negative_temperature#Lasers

Re: Can you use a magnifying glass and moonlight to light a fire? (2016)

#34

Post by nickparker »

parliament32 wrote: post #12

> You can't use lenses and mirrors to make something hotter than the surface of the light source itself. This is an interesting argument. Can I not reflect some sunlight off a mirror, then do the magnifying-glass-to-start-a-fire trick in daytime? Doesn't the mirror stay cool? Isn't the moon just a (poor) mirror for the sun's light?

You've found the right question to ask.

Your mirror in sunlight works because the reflectivity or albedo of the mirror is very high relative to whatever target you're lighting on fire.

In a magical closed system where radiative heat transfer was the only factor, objects of differing reflectivity would eventually reach temperature equilibrium through black body radiation.

We aren't interested in closed systems though. The moon's temperature is set by the equilibrium between incident solar radiation and black body emission, most of which flies off into deep space making the system very open. Just like your mirror's temperature is the equilibrium of incident radiation, black body cooling, and convective cooling in Earth's atmosphere.

If the moon had a high reflectivity and/or a powerful cooling mechanism like convection, its equilibrium temperature would be far lower than the temperature of its emitted + reflected light. Unfortunately the moon's albedo is just 0.12 and black body radiation is all it's got, so the modest difference between its temperature and that of its light isn't enough to start a fire.

Re: Can you use a magnifying glass and moonlight to light a fire? (2016)

#35

Post by baddox »

nealabq wrote: post #27
parliament32 wrote: post #12

> You can't use lenses and mirrors to make something hotter than the surface of the light source itself. This is an interesting argument. Can I not reflect some sunlight off a mirror, then do the magnifying-glass-to-start-a-fire trick in daytime? Doesn't the mirror stay cool? Isn't the moon just a (poor) mirror for the sun's light?

If the light is being radiated from a black body surface, then you cannot make something hotter than that surface. But you're right, the light from the moon is reflected sun light, plenty hot to start a fire. The moon also radiates like a black body, but virtually all that light has longer wavelengths than the reflected visible light. You could start a fire from the light of a single star if you had a big enough lens…

> But you're right, the light from the moon is reflected sun light, plenty hot to start a fire.

That's my impression as well. A blackbody has albedo 0. The moon has an albedo of around 0.12. While I suspect you can't start a fire from moonlight in practice, I don't think the arguments in this article are correct.

Re: Can you use a magnifying glass and moonlight to light a fire? (2016)

#36

Post by [deleted] »

FiatLuxDave wrote: post #17

I love xkcd, but this is completely wrong. It is well known that the spectral temperature of the moon is about 4000K. See for example : http://www.lumec.com/newsletter/architect_06-10/the_sun_the_... or https://physics.stackexchange.com/questions/244922/why-does-... . That is the maximum temperature that you can achieve with light from the moon, no matter how concentrated. 4000 K is plenty hot enough to start a fire.

[deleted]

Re: Can you use a magnifying glass and moonlight to light a fire? (2016)

#37

Post by pierrebai »

wcoenen wrote: post #23
DenisM wrote: post #14

Thermodynamics argument seems iffy to me. I can cover the entire surface of the earth with solar panels to harvest the moon light and use the combined electricity to melt iron. If this is ok with thermodynamics then so is using lenses. The rest of the argument seems to be that light cannot be optically condensed to a single point as there will always be some dispersion due to diffraction, and the size and shape of th…

> I can cover the entire surface of the earth with solar panels to harvest the moon light and use the combined electricity to melt iron. If this is ok with thermodynamics then so is using lenses. There's a difference. By going through solar panels you would increase entropy because the conversion of sunlight to electricity can't be 100% efficient[1]. And if you allow the solar panels to heat up too much because of th…

The region of higher temperature is much smaller. Entropy is not a pure measure of energy or randomness, but a measure of that within a volume. You'd have smaller entropy in a small volume, but higher around it because you would have diverted the ray that would have hit outside the concentrated region.

Were is this reasoning wrong?

Re: Can you use a magnifying glass and moonlight to light a fire? (2016)

#38

Post by zeristor »

So what about non-linear optics and optical frequency doubling?

https://en.wikipedia.org/wiki/Second-harmonic_generation

It was just an option on my MSc in LASERs but I thought it was cool (with the potential to be very warm).

Although my frequency has been halved and I've been working in software for decades

Re: Can you use a magnifying glass and moonlight to light a fire? (2016)

#39

Post by whatshisface »

DenisM wrote: post #26
schoen wrote: post #20

Earlier quoted context omitted.

I was wondering about this too. After all, human beings have produced temperatures that were hotter than the sun's surface and almost all of the energy that we used to do that ultimately came from the sun (except for very small components that came from nuclear reactions on Earth and geothermal energy, which also comes from nuclear reactions inside Earth). I think that this issue is addressed by footnote 2 > And, mor…

I can convert the optics to non-reversible by spreading a thin layer of dust on top. Now both systems are non-reversible and both allow higher temperatures? Or are we looking at a particular kind of non-reversibility?

Covering the lens with dust would let you light a fire. The dust will warm slightly above ambient temperature in the lens' shadow, so you can use it to power a Carnot engine that winds up a spring that is then released to rub a twig against a log.

Re: Can you use a magnifying glass and moonlight to light a fire? (2016)

#40

Post by DuskStar »

jstanley wrote: post #9
randyrand wrote: post #7

Earlier quoted context omitted.

What if the sun was a bunch of parallel laser beams? Then you could focus them all into a single point.

Right, but that point would be exactly as hot as the surface that the light was coming from, and no hotter. (Is the argument as I understand it).

Wouldn't that kind of make the National Ignition Facility [0] impossible?

0: https://en.wikipedia.org/wiki/National_Ignition_Facility

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