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)

#131

Post by baddox » Sat, Dec 22, 2018, 4:07 AM UTC

analog31 wrote: post #129

Here's a thought experiment to consider. Imagine creating a lens to focus the blackbody radiation from a stack of bricks onto a single brick, and heating up the brick. Now focus the light back onto just a small region of the brick pile, and heating up that region, which in turn heats the rest of the bricks by conduction. This in turn increases the amount of heat collected from the pile, ad infinitum or until the bric…

The problem is that some of the radiation from the moon is not blackbody radiation.

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

#132

Post by twodave » Sat, Dec 22, 2018, 4:10 AM UTC

DenisM wrote: post #94

08-15 wrote: post #89

Earlier quoted context omitted.

I'll repeat for the reflexive downvoters: Solar panels will work in moonlight if and only if you can make fire from moonlight with a magnifying glass. The answer depends on how good a mirror the moon is. It calls for a real experiment, not a thought experiment. I don't really know which way it will go.

You're not addressing the essential part of my comment. Let me clarify: I put in place 1000 solar panels and aim them at the sun thus procuring 200kw of power. Next I use it to power an arc welder, producing 10,000 C of heat. Therefore I have used a 6,000C sun to produce 10,000C on earth. Would a similar setup work with the moon? I don't know. That was not my point, my point was that it certainly is possible to produ…

Sure it is, just not in real-time. The lense fails because there is no storage mechanism.

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

#133

Post by URSpider94 » Sat, Dec 22, 2018, 4:16 AM UTC

DenisM wrote: post #109

Gatsky wrote: post #100

Earlier quoted context omitted.

Reversible means you don't increase the entropy of the system. If you relax this restriction, you can do useful work, like powering an arc welder or growing carrots to feed yourself and rubbing sticks together to start a fire. Merely making the lens non-reversible doesn't mean it will suddenly set things on fire.

How does it work in practice? I am dumping some amount of low-energy photons onto a target, the target is heated up and radiating out the same amount of energy it receives. As I increase the number of photons hitting the target its temperature raises and it radiates more heat outwards, shedding excess heat. Then as I keep increasing the number of photons, the temperature stops increasing at some point having reached…

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 photons, then the focused image just grows by a corresponding amount.

Optics (and thermodynamics) tell us that we can not generate a higher flux of photons into a target area, than left the equivalent area at the source. Assuming that both the source and target are black body radiators, that says the target can’t get hotter than the source.

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

#134

Post by URSpider94 » Sat, Dec 22, 2018, 4:22 AM UTC

DenisM wrote: post #94

08-15 wrote: post #89

Earlier quoted context omitted.

I'll repeat for the reflexive downvoters: Solar panels will work in moonlight if and only if you can make fire from moonlight with a magnifying glass. The answer depends on how good a mirror the moon is. It calls for a real experiment, not a thought experiment. I don't really know which way it will go.

You're not addressing the essential part of my comment. Let me clarify: I put in place 1000 solar panels and aim them at the sun thus procuring 200kw of power. Next I use it to power an arc welder, producing 10,000 C of heat. Therefore I have used a 6,000C sun to produce 10,000C on earth. Would a similar setup work with the moon? I don't know. That was not my point, my point was that it certainly is possible to produ…

Monroe’s explanation only applies to optical systems, ie reflecting and focusing light, which is a thermodynamically reversible process (yes, even with dust. Dust doesn’t make the lens thermodynamically irreversible, it just scatters and absorbs some of the light so it doesn’t reach the lens. The irreversibility is a property of the lens). Solar panels are converting light energy into electrical energy, which is not an irreversible process - it creates entropy, and therefore the rules are different.

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

#135

Post by tigershark » Sat, Dec 22, 2018, 4:23 AM UTC

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. (…

There is no need for an experiment, the moonlight does produce energy in a solar panel. Obviously the power produced will be much much lower than with the sun. I remember that as a child I was able to use a solar calculator indoor with just the power gathered from an incandescent light.

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

#136

Post by URSpider94 » Sat, Dec 22, 2018, 4:24 AM UTC

aasasd wrote: post #102

> Lenses and mirrors work for free; they don't take any energy to operate. Wait a minute. Does that mean that I could get a tiny solar panel and light it up with a lens, instead of getting big panels? Energy output of a panel is proportional to the amount of light that hits it, right? I guess that lenses are ‘free’ only if they have no impurities, but even then, assuming solar panels are costlier than plastic lenses,…

There are companies that do this. They build tiny solar panels out of materials that can withstand high junction temperatures, and use fresnel lenses to concentrate light onto them.

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

#137

Post by dreamcompiler » Sat, Dec 22, 2018, 4:42 AM UTC

crazygringo wrote: post #81

Wow... separately I had no idea the surface of the moon reaches (and goes above) 100°C... that's hot ! Literally boiling hot. Turns out at night it goes down to almost –200°C. That's insane . Quick searching of how the astronauts survived this, turns out it seems they timed landings to the lunar dawn for an in-between temperature, that the lunar surface doesn't conduct heat well (all dust?), of course there's no atom…

Earth would have the same temperature extremes without the atmosphere to moderate them. Artificial satellites also have to deal with this; there are hundreds of degrees variation when the satellite is in sunlight vs the Earth's shadow. And even between the shadow side of the satellite itself vs the sunlit side.

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

#138

Post by dekhn » Sat, Dec 22, 2018, 4:42 AM UTC

It is a truism: no matter how many times this is explained, some engineer will come up with a complicated system that violates the laws of thermodynamics and refuse to admit that their idea is extremely unlikely. The laws of thermo are some of the best understood and most well-supported physical systems that humans have yet invented. Every time somebody comes up with a perpetual motion machine, it gets shot down because the person who invented it literally ignored all the really well-understood math and physical theory in thermo.

it's like there's a brain bug where engineers think they can outsmart 200+ years of scientific progress with a clever arrangement of mirrors.

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

#139

Post by nkurz » Sat, Dec 22, 2018, 5:08 AM UTC

dekhn wrote: post #138

It is a truism: no matter how many times this is explained, some engineer will come up with a complicated system that violates the laws of thermodynamics and refuse to admit that their idea is extremely unlikely. The laws of thermo are some of the best understood and most well-supported physical systems that humans have yet invented. Every time somebody comes up with a perpetual motion machine, it gets shot down beca…

It is a truism: no matter how many times this is explained

What exactly is the "this" that you refer to? I don't think the issue is that the "engineers" disagree with the physical principles, rather they tend to disagree that the physical principles apply in quite the way that the author claims. Many of the engineers probably believe is that Munroe is correct in claiming that on cannot start a fire with a low temperature blackbody radiation source regardless of the size of one's magnifying glass, but disagree that it is reasonable to consider sunlight reflected by the moon as fitting this model.

Presumably, you agree that it's possible to start a fire using a magnifying glass using sunlight on earth. I'd guess you also believe that it's possible reflect the light from small handheld mirror into the magnifying glass, and still start a match, even though the mirror is much lower than the temperature of the sun? While the specular reflection from the mirror is different than the diffuse reflection from the moon, one might note that the words "specular" and "diffuse" don't appear in Munroe's exposition. Would you agree that Munroe's argument would appear to prohibit this behavior?

Now assume that the moon was replaced by an equally sized parabolic mirror aimed to be focused on the earth. Would it be possible to light a match using this light if one's magnifying glass was large enough? Which thermodynamical principle am I violating in thinking that it might be possible? And which part of Munroe's argument do I invalidate by making these modifications? Again, my point isn't that Munroe's conclusion is wrong, just that there might be something flawed about the argument he uses to reach that conclusion. This might be a "brain bug", but from the inside it just feels like an attempt to understand truth.

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

#140

Post by amluto » Sat, Dec 22, 2018, 5:21 AM UTC

sandworm101 wrote: post #115

tbabb wrote: post #112

Earlier quoted context omitted.

Sorry, this is wrong. Please don't "explain" your misunderstanding as if you know the answer; ask questions instead. If we imagine a full sphere of inward-pointing fibers, each one "looking" at the moon, then we see moon-surface in all directions from within this contraption. We are in a thermal bath of moon-temperature. We will not get hotter than the moon. And fibers are not needed to create this circumstance. The…

Not the system i was talking about. None of the fibers would "look at the moon". Fiber isnt the same as lenses. Note too that i said match. Getting tiny bit of a matchhead hot enough to decompose isnt the same as burning an ant. A matchhead on the surface of the moon would probably ignite just fine (150*+o2).

Fiber is very much the same as lenses.

(Well, the fiber you’re thinking of is. Fluorescent fiber is not, but that’s another story.)