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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)

#51

Post by schoen »

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?

I think the thermodynamics argument is that only non-reversible systems cause energy to flow from a lower-temperature source to a higher-temperature destination, not that all non-reversible systems do so. For example, if you set up a solar panel that directly feeds a resistor, you have a dissipative system that's unlikely to get hotter than the sun. However, a solar panel array and/or battery could conceivably power a fusion reaction that does get hotter than the sun.

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

#52

Post by Isobit »

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?

Wouldn't this layer of dust also stop the lense from working normally? If I understand the other commenters here, it's not just about reversibility, but "useful energy" loss - with lenses you are not paying the needed "efficiency tax" to achieve higher temperatures than the source.

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

#53

Post by IgorPartola »

I don’t buy the argument that you can’t concentrate two beams on the same spot. Sure you might not be able to do that with one lens, but concentrating on a small area is as good an approximation. But if you require that it really be a point, why can’t I do that with N lenses and mirrors that are fully reversible but all align to aim at the same point from different angles?

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

#54

Post by dr_orpheus »

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.

Color temperature is a representation of the shape of the spectrum as compared to a blackbody of the same temperature. It is not necessarily representative of the surface emitting the light. Because the moon absorbs some of the wavelengths of light more than others it shifts the spectrum to represent a blackbody object of a lower temperature. So depending on what wavelengths are absorbed the color temperature can actually be shifted up or down.

For another example, the color temperature of standard incandescent bulbs is ~2700K, and this is similar to blackbody radiation. However, LED lights can have the same color temperature, but the surface of the LED is not at 2700K because the light emitted is being created through a different process than blackbody radiaton.

That said, I am still wrapping my head around the xkcd explanation because blackbody radiation of the moon at 100K would not be enough but there is also reflected sunlight at a .16 albedo.

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

#55

Post by schoen »

robertelder wrote: post #48
landryraccoon wrote: post #31

Earlier quoted context omitted.

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.

I don't know very much thermodynamics, but I'm not sure that thermodynamic temperature is well-defined for a system consisting of "just a few atoms".

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

#56

Post by stcredzero »

baddox wrote: post #45
stcredzero wrote: post #30

Earlier quoted context omitted.

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 pr…

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?

To get past the diffraction limits, those "few small perfect mirrors" on the Moon would have to be gargantuan. (If I'm remembering correctly, they'd be the size of the alien space ships in Star Trek that make the Enterprise-D look teeny.) Otherwise, viewed from the surface of the Earth, those mirrors would just constitute a minuscule brightening of the sky in the direction of the Moon.

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

#57

Post by stcredzero »

saagarjha wrote: post #44
stcredzero wrote: post #30

Earlier quoted context omitted.

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.

So it's approximately a black body. Even asphalt has tiny bits of specular reflection coming off of it.

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

#58

Post by btilly »

nickparker wrote: post #34
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.…

Really?

According to the Stefan-Boltzmann Law, the energy put out in blackbody radiation is proportional to temperature to the 4th power. Therefore something that is 5000 degrees reflected off of an object with albedo 0.12 is putting out 0.12 times the energy it originally did, while something that is 2500 degrees only puts out (1/2)^4 = .0625 times as much. So the "temperature of the Moon's light" should be more than hot enough to light something on fire if it is focused right.

As a sanity check, compare how much light the moon puts out as a black body in shadow with what it reflects from the Sun. As another sanity check, compare how bright the Moon is versus a fire.

What am I missing here?

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

#59

Post by hinkley »

IgorPartola wrote: post #53

I don’t buy the argument that you can’t concentrate two beams on the same spot. Sure you might not be able to do that with one lens, but concentrating on a small area is as good an approximation. But if you require that it really be a point, why can’t I do that with N lenses and mirrors that are fully reversible but all align to aim at the same point from different angles?

Don't all the biggest telescopes on earth use multiple reflectors these days?

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

#60

Post by jstanley »

DuskStar wrote: post #40
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).

Wouldn't that kind of make the National Ignition Facility [0] impossible? 0: https://en.wikipedia.org/wiki/National_Ignition_Facility

From what I've read in the other comments, the argument doesn't apply to lasers.
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