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

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

#161

Post by nshepperd »

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?

The thermodynamic argument applies to black bodies, and says that you can't make an object hotter than the surface of the emitter. That's pretty uncontroversial.

The more general argument based on etendue, which applies to the moon, is: you can't make the incoming light any brighter than it is on the surface of the source (which doesn't have to be the original emitter, but can be any point along the path of the light). As a corollary this happens to mean you can't really make something hotter than a rock on the moon.

Suppose the moon was actually a flat mirror. Standing on the mirror-moon, you look at the ground. In most directions you would see the darkness of space (with a few reflected stars), but in one spot you would see about ~(0.5°)^2 solid angle of extreme brightness - the sun's reflection in the mirror. Standing on the mirror-moon, you could certainly use a magnifying glass to heat something to ignition temperature (ignoring the lack of oxygen) by making its environment that bright using the reflected light.

Similarly, if the moon was a mirror, you could certainly use the reflected light to start a fire standing on earth (provided you're lucky enough for the moon/earth/sun to line up just right so that the reflection of the sun is visible through small angle subtended by the moon from earth). It would basically appear as another sun in the sky when lined up properly.

In reality, instead of reflecting light like a mirror, the moon scatters light in all directions. Standing on the actual moon, looking at the ground, what you see is a lot of moon dirt, all of which about equally bright (ie. not very). The scattering smooshes the sun's light out in all directions, ensuring there's no visible "reflection" of the sun in any direction.

The most you can do with this scattered moon light is make the environment of an object as bright as the (mediocre) brightness of the moon rocks. But moon rocks already experience that environment of mediocre brightness, and reach only 100°C, so you won't be able to make your object much hotter than that.

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

#162

Post by URSpider94 »

Drury wrote: post #148
URSpider94 wrote: post #133

Earlier quoted context omitted.

The point is, you can never get to the point where you are dumping more photons in to make it hotter, because there are not more photons. There is a limit to how small you can focus a lens. Once the object you are focusing is in sharp focus, it will start to go out of focus again if you go past that point. So you can’t just arbitrarily increase the photon density further. If you make your lens bigger to gather more p…

Consider this: Say a light source has a T temperature resulting in X photons emitted. I redirect all the photons to a single point. I see arguments mentioning that that single point cannot be hotter than the source because there's no more photons to make it hotter. I now add a second light source of the same T temperature, that emits the same amount of photons and also focus all of them on that same point. I now have…

In order to truly move all of the photons from the emitter to the target, you need to effectively surround the target with lenses, such that it looks from the surface of the target as if the emitter is filling the sky.

If that is the case, then there is no extra room to fit the optics to focus another source onto the target.

If you shrink the optics enough to make room for another source, then you also aren’t delivering all of the photons from the original source (you can’t be, since you aren’t covering all incoming angles), and therefore it’s not the same temperature as the source.

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

#163

Post by Dylan16807 »

Drury wrote: post #148
URSpider94 wrote: post #133

Earlier quoted context omitted.

The point is, you can never get to the point where you are dumping more photons in to make it hotter, because there are not more photons. There is a limit to how small you can focus a lens. Once the object you are focusing is in sharp focus, it will start to go out of focus again if you go past that point. So you can’t just arbitrarily increase the photon density further. If you make your lens bigger to gather more p…

Consider this: Say a light source has a T temperature resulting in X photons emitted. I redirect all the photons to a single point. I see arguments mentioning that that single point cannot be hotter than the source because there's no more photons to make it hotter. I now add a second light source of the same T temperature, that emits the same amount of photons and also focus all of them on that same point. I now have…

> I redirect all the photons to a single point.

For that to be possible with a blackbody light source, it has to itself be a single point. Which means a temperature of approximately infinity.

For a real light source, one that has an area, you can at best focus it down to the same area. To get maximum light to a target, you either have to make the target almost touch the source, or you have to have it so no matter what direction you go from the target, you hit a lens that's bringing light from the source. Once you set either one of those up, there's nowhere to fit a new light source.

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

#164

Post by ummonk »

bscphil wrote: post #50
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…

Yeah, it does contradict that. You can't really "bundle" all the light from the sun like that and aim it at the Earth without violating thermodynamics. The way to do it would be something like wrapping the sun and the Earth together in a giant chamber made out of some perfect mirror (actually maybe the mirror isn't necessary), with the only exit being the dark side of the Earth. And that would heat up both the Earth and the Sun to millions of degrees.

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

#165

Post by ummonk »

08-15 wrote: post #43
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.)

Yes, this is what would happen.

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

#166

Post by speedplane »

Dylan16807 wrote: post #163
Drury wrote: post #148

Earlier quoted context omitted.

Consider this: Say a light source has a T temperature resulting in X photons emitted. I redirect all the photons to a single point. I see arguments mentioning that that single point cannot be hotter than the source because there's no more photons to make it hotter. I now add a second light source of the same T temperature, that emits the same amount of photons and also focus all of them on that same point. I now have…

> I redirect all the photons to a single point. For that to be possible with a blackbody light source, it has to itself be a single point. Which means a temperature of approximately infinity. For a real light source, one that has an area , you can at best focus it down to the same area. To get maximum light to a target, you either have to make the target almost touch the source, or you have to have it so no matter wh…

> For a real light source, one that has an area, you can at best focus it down to the same area.

I'm not an optics expert, but this can't be true. You can clearly focus light emitting from an area into a smaller area (although probably not to an infinitesimally small area).

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

#167

Post by nkurz »

ummonk wrote: post #159
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?

This works because you're just concentrating the image of the sun. It's the surface temperature of the sun that matters. Now if, instead, the mirror were scattering or absorbing and re-radiating the light (as the moon does), then its temperature would be limiting factor.

Now if, instead, the mirror were scattering or absorbing and re-radiating the light (as the moon does)

Are you saying that mirrors do something other than absorbing and re-radiating light? In most explanations I've seen, this is exactly what they do:

How does the mirror reflect light? The silver atoms behind the glass absorb the photons of incoming light energy and become excited. But that makes them unstable, so they try to become stable again by getting rid of the extra energy—and they do that by giving off some more photons. (You can read about how atoms take in and give out photons in our article about light.) The back of a mirror is usually covered with some sort of darkly colored, protective material to stop the silver coating from getting scratched, and also to reduce the risk of any light seeping through from behind. Silver reflects light better than almost anything else and that's because it gives off almost as many photons of light as fall on it in the first place. The photons that come out of the mirror are pretty much the same as the ones that go into it.

https://www.explainthatstuff.com/howmirrorswork.html

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

#168

Post by nshepperd »

skolos wrote: post #145

Interesting article. Right in many places. Wrong (possibly) in main conclusion. Entropy argument - correct in the sense that using radiation from black body we cannot use lenses to heat another body to the temperature higher than original. Easy to understand why - the first body has a temperature, radiation has the same temperature, if we apply the radiation to another object it will not heat up more than the radiati…

The conclusion is correct. It doesn't rely on the moon being a black body radiator, but on etendue, which shows that the most you can do with a system of lenses and mirrors is to create an environment (on earth) as bright as the environment on the moon. The fact that rocks on the moon's surface only reach 100°C shows that an environment like that is not bright enough to light a fire.

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

#169

Post by URSpider94 »

hinkley wrote: post #66

Isn't the bigger problem that sunlight is nearly parallel and moonlight is reflected off of a spherical surface? How are you violating conservation of energy if you're taking all of the light that would hit a square mile of the earth and concentrating it down to the size of a penny? If you can't concentrate light that way then how do focusing lenses on cutting lasers function? Makes no sense.

Indeed, you can not focus all of the sunlight that would hit a square mile of Earth down to a penny. You just can’t. When you focus the sun, you’re not making an infinitely small spot, you are making a tiny image of the sun. The bigger your lens, the larger that image. You can’t get any more focused than “in focus”.

When you focus a cutting laser, you are imaging the shape of the laser cavity. The emission is coming from a very narrow spatial region, so you can focus it back down to a small spot. However, a laser that is out of alignment, will often not be able to be focused to a small spot.

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

#170

Post by nkurz »

AlexCoventry wrote: post #141
wrycoder wrote: post #122

Earlier quoted context omitted.

The point is interesting. A CO2 laser at 10 microns can melt steel, which emits a whole lot of 0.5 micron photons. The problem with sunlight reflected off the moon is that it's like having a very low transmittance ND filter in the optical path. There aren't a whole lot of photons entering the aperture, and as the material begins to heat, there are more photons and overall probably more energy going out. (I'm not givi…

What's ND?

Neutral Density: https://en.wikipedia.org/wiki/Neutral-density_filter
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