Can you use a magnifying glass and moonlight to light a fire? (2016) • Hacker News Board

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

#21

Post by antognini » Fri, Dec 21, 2018, 10:44 PM UTC

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 difference is that a lens is a reversible system, whereas using energy from solar panels to melt iron is not. If you don't have a reversible system it is perfectly possible to take energy from a cold object and make a hot object hotter, but you cannot do this reversibly.

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

#22

Post by saagarjha » Fri, Dec 21, 2018, 10:47 PM UTC

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…

Solar panels lose energy to heat in this case, so entropy does not decrease.

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

#23

Post by wcoenen » Fri, Dec 21, 2018, 10:49 PM UTC

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 that waste heat, then that efficiency will drop further.

If a lens could heat a point to a temperature higher than the sun, then there'd be no such loss and you'd be magically decreasing entropy.

[1] https://en.wikipedia.org/wiki/Thermodynamic_efficiency_limit

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

#24

Post by adammunich » Fri, Dec 21, 2018, 10:49 PM UTC

I'm not so sure about this.

Imagine you had a large cloud of planar mirrors, each, specifically can be aimed at any given point --even points that overlap.

While I agree that you cannot focus a whole image to a smaller area than the diffraction limit allows for a continuous lens surface, if you omit diffraction, mirrors could certainly do it.

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

#25

Post by saagarjha » Fri, Dec 21, 2018, 10:50 PM UTC

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.

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

#26

Post by DenisM » Fri, Dec 21, 2018, 10:50 PM UTC

schoen wrote: post #20

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 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?

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

#27

Post by nealabq » Fri, Dec 21, 2018, 10:59 PM UTC

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. You could also start a fire with the light from a hand mirror at the distance of the moon if it reflected the sun's light at you. But you'd need a very big lens.

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

#28

Post by dr_orpheus » Fri, Dec 21, 2018, 10:59 PM UTC

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…

You are correct about the solar panels. Solar panels are more efficient when they are colder, i.e. the temperature delta between the sun and the panel is larger.

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

#29

Post by jplee » Fri, Dec 21, 2018, 10:59 PM UTC

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).

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...

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

#30

Post by stcredzero » Fri, Dec 21, 2018, 11:00 PM UTC

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 system can achieve is much less that your mirror/magnifying glass system. A perfect black body radiator is just about the poorest mirror there is. The moon's light isn't what you'd get from a perfect black body. However, it's a long, long ways away from being a perfect mirror and probably far closer to being a black body.