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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)

#41

Post by whatshisface »

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...

Like the other commenter said, lasers are not black-body emitters. They "pump" light out and are very far from equilibrium.

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

#42

Post by whatshisface »

randyrand wrote: post #7
jstanley wrote: post #4

Earlier quoted context omitted.

I think TFA refutes your argument, but I'm not smart enough to know for sure. Can you please expand on this?

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

Lasers are kept far from equilibrium by design, unlike blackbody radiators which are in equilibrium with the electromagnetic field they are radiating in to.

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

#43

Post by 08-15 »

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

Are you sure about that? First off, take an off-the-shelf solar panel, point it at the moon in the middle of the night. You get a grand total of nothing. Okay, it was a cheap panel, that might not generalize to anything.

But more importantly, by the argument laid out in the article, your solar panels cannot work in moon light. (Or maybe they work at horrible efficiency, because the moon is a bit warmer than the earth.) I'm not sure I buy that argument; maybe you should run the experiment.

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

#44

Post by saagarjha »

stcredzero wrote: post #30
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?

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? The mirror is a part of you hybrid reflective/refractive light concentration system. Of course it stays cool, otherwise your system is inefficient. Doesn't the mirror stay cool? Isn't the moon just a (poor) mirror for the sun's light? Precisely. So the maximum temperature that sys…

The moon is about as reflective as asphalt.

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

#45

Post by baddox »

stcredzero wrote: post #30
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?

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? The mirror is a part of you hybrid reflective/refractive light concentration system. Of course it stays cool, otherwise your system is inefficient. Doesn't the mirror stay cool? Isn't the moon just a (poor) mirror for the sun's light? Precisely. So the maximum temperature that sys…

> So the maximum temperature that system can achieve is much less that your mirror/magnifying glass system.

Why? If the moon were a perfect black body, but with a few small perfect mirrors on the surface to bounce a bit of sunlight towards Earth, couldn't we (in theory) focus that tiny bit of sunlight to heat something up to the temperature of the Sun? There may not be enough solar radiation to make it feasible in practice, but it seems like you need to actually do the math instead of just saying "the moon's surface temperature isn't high enough to start a fire, Q.E.D."

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

#46

Post by sampo »

marcinzm wrote: post #19
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…

>If this is ok with thermodynamics then so is using lenses. It's okay because solar panels are not perfectly efficient while lenses are, in theory, perfectly efficient. That efficiency loss is in essence the "cost" of moving heat from a colder to a hotter place. I'm guessing, but don't quote me on it, that solar panel efficiency is related to the temperature of the sun and the local environment just like any other he…

> solar panel efficiency is related to the temperature of the sun and the local environment just like any other heat engine

You are correct, https://en.wikipedia.org/wiki/Solar_cell_efficiency#Thermody...

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

#47

Post by newnewpdro »

I'm pretty sure if I were to go pick up a number of magnifying glasses and focus them on the same point using moonlight, the temperature at that point would increase with every additional magnifying glass.

Am I to accept that the additional magnifying glasses would cease increasing the temperature once the temperature matched that of the moon's surface?

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

#48

Post by robertelder »

landryraccoon wrote: post #31
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.

If the size of the penny was reduced to just a few atoms, wouldn't changing the number of atoms in the penny affect the rate at which the penny itself could emit radiation through blackbody emission? If so, I would expect there would be a size at which the penny could be small enough that it's temperature could grow arbitrarily large once the heat flux in = heat flux out from the penny.

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

#49

Post by thatcherc »

saagarjha wrote: post #25
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.

Randall has a response to this argument in the article.

Does he though? Here's all I see -

> "But wait," you might say. "The Moon's light isn't like the Sun's! The Sun is a blackbody—its light output is related to its high temperature. The Moon shines with reflected sunlight, which has a "temperature" of thousands of degrees—that argument doesn't work!"

> It turns out it does work, for reasons we'll get to later.

But the rest of the article is about etendue, and I don't see how the issue of reflected light is addressed (though possibly the answer is implied with an etendue argument I missed).

I'm very curious now - it seems to me that if the Moon was a perfect mirror, you should be able to start a fire with it. Maybe the Moon's low albedo is the reason? Fun fact about the Moon: its albedo (roughly the fraction of incident light it scatters) is about the same as asphalt - not very reflective at all! [0] It just looks bright to us because the Sun is so tremendously bright.

[0] - https://www.lcas-astronomy.org/articles/display.php?filename... , also learned this in an astronomy class

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

#50

Post by bscphil »

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…

The interesting thing to me is that Randall describes a very similar system just a few What-Ifs earlier: https://what-if.xkcd.com/141/

Here he says very clearly that if you "bundled" all the light from the sun and aim it at the earth it would heat the atmosphere to millions of degrees (the surface of the sun is much less than that). It's not at all clear to me what he means by "bundled" and why it's not contradictory to what he says in the article here. Presumably some kind of lens / mirror system could be used?

It seems to me that in this article he has in mind some highly abstract system that's fully reversible. Of course, in that case, once the target object gets hot enough it will start emitting light and result in equilibrium. But it's not clear to me that this describes what would actually happen with a real optical system! E.g. (a) much of the light the target receives is going to be absorbed and reemitted away from the lens, (b) what if you removed the lens targeting system at the precise moment the light impacted the target, so that the system couldn't be reversed, etc.

Edit: one more thing. The surface of the lit side of the moon can reach 260 degrees F, and dry wood can potentially catch fire as low as 300 degrees F. And the moon has some reflectivity as well. So even taking Randall's claims on their face, I'm skeptical that you could not start a fire (in some materials at least) using moonlight.

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