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)

#11

Post by robertelder » Fri, Dec 21, 2018, 10:22 PM UTC

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 image shown in the article). If you consider conservation of energy for the energy flux from the surface of the sun being entirely directed to a single body of matter that absorbs this heat (assume it's a penny), the steady-state blackbody emission of the penny would have to equal the energy flux from the entire surface of the sun. I think this situation would end up making the 'temperature' of the penny much higher than the surface of the sun for the same reason that the center of the sun is hotter than the surface: There is energy expended, and it comes from the fusion of light elements inside the sun, so there is no violation of entropy as stated in the article: "you'd be making heat flow from a colder place to a hotter place without expending energy."

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

#12

Post by parliament32 » Fri, Dec 21, 2018, 10:24 PM UTC

> 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?

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

#13

Post by aqme28 » Fri, Dec 21, 2018, 10:28 PM UTC

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…

You cannot use a magnifying glass to make something hotter than the source of the light. The thermodynamics principle is unyielding.

This is a tough tricky thermodynamics question because it really seems like you can use a really big magnifying glass to make say an object that is hotter than the sun. I remember working through it in a physics class.

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

#14

Post by DenisM » Fri, Dec 21, 2018, 10:31 PM UTC

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 the dispersion is dictated by that of the source. That is you can't make the target denser then source, hence the temperature must be less.

EDIT: Several commenters submitted that lenses are reversible while solar panels are not, and this makes all the difference. My retort is that I can make non-reversible lenses by covering them in a thin layer of dust. Since the lense system is now non-reversible can I use these sub-par lenses to create higher temperature than I could with clear lenses?

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

#15

Post by dooglius » Fri, Dec 21, 2018, 10:32 PM UTC

One thing that makes me uncertain about this is the fact that the sun is generating the light from a fusion reaction, thus expending energy. In other words, the system entropy does not decrease because the fusion reaction makes up for any lost entropy by the cold-to-hot temperature flow. This is the same reason why a system consisting of a battery and a fridge would work.

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

#16

Post by wefarrell » Fri, Dec 21, 2018, 10:33 PM UTC

I've often wondered how big of a lens you would need to grow a plant using a light source from outside of the solar system, if it's even possible.

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

#17

Post by FiatLuxDave » Fri, Dec 21, 2018, 10:33 PM UTC

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.

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

#18

Post by JimboOmega » Fri, Dec 21, 2018, 10:33 PM UTC

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…

If you directed all the input to the penny, then you've created a close system that can't lose energy, and doesn't the whole system - including the sun - keep rising in temperature anyway?

Ultimately the outside of this inverted sun system needs to vent the whole of the energy, the energy can't just fall inwards.

What you're describing is a reversal though; it requires the penny to be much hotter than the sun, only as a result of the sun's energy flowing into it.

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

#19

Post by marcinzm » Fri, Dec 21, 2018, 10:42 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…

>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 heat engine.

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

#20

Post by schoen » Fri, Dec 21, 2018, 10:42 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 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, more specifically, everything [lenses and mirrors] do is fully reversible—which means you can add them in without increasing the entropy of the system.

Presumably we can't say the same of the electrical devices that we use to collect, store, and transmit sunlight, or to create high temperatures from this stored energy. For example, if you use a "lunar panel" to store energy in a battery and then heat something on an electric stove, many of the components in this process will not be reversible, differently from mirrors and lenses. (Putting a hot object on top of the stove won't cause the lunar panel to emit light back in the direction of the moon!)

So I think footnote 2 is actually very important, because it's not that we can never use any energy source to create something hotter than that source, it's that we can never do so using only reversible processes, including purely passive optics.