And what of materials that burn at a temperature below the temperature of the Moon? If 100C is the limit, there are materials that burn at much lower temperatures such as Phosphorous (34C).
Can you use a magnifying glass and moonlight to light a fire? (2016)
121–130 of 277 posts
Re: Can you use a magnifying glass and moonlight to light a fire? (2016)
#122Earlier quoted context omitted.
All photons can start fires. They arent hotter or colder individual photons, just photons at different energy levels/wavelengths/relative velocity. Heat happens once photons collide with stuff. Heat is a group effort. Get enough photons to hit something and it will warm. Photon colour, and the reflectivity of the struck object, alters the needed number but with infinite photons fire (300*?) is always possible. Fiber…
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…
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 giving an answer here, just some thoughts.)
Re: Can you use a magnifying glass and moonlight to light a fire? (2016)
#123Earlier quoted context omitted.
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 tha…
The angular concentration of the light. The moon is diffuse, so an incoming ray of sunshine is spread by the optically rough surface of the moon from an incident solid angle of 6 * 10^-5 steradians out into 2 pi steradians of the night sky, or a reduction in angular concentration by a factor of about 100,000 (totaling ~1 million after the albedo is accounted for). It is like you are looking at the sun through a mirro…
This is the primary reason. You can no more start a fire with sunlight reflected off the moon at night than with sunlight reflected off a sheet of paper during the day. (In fact you can do better with the sheet of paper, because you could in theory surround it with lenses to recapture the diffuse light.)
Re: Can you use a magnifying glass and moonlight to light a fire? (2016)
#124Earlier quoted context omitted.
Making solar cells that effectively can handle concentrated sun light is difficult (if they heat up efficiency goes down, ...), so it's easier to just fill an area with solar cells instead of with mirrors pointing at a smaller cell area.
> if they heat up efficiency goes down So you're saying I should cool them with water and use the steam to move turbines, then I'm golden! \($ ∇ $ )/
I'm not sure if you are joking or not, but something like this is still used for solar power generation.
Re: Can you use a magnifying glass and moonlight to light a fire? (2016)
#125So thinking about this some more, the argument here is that if you use “just” a magnifying glass of arbitrary size and shape you can’t do this. I can buy that based on the arguments presented. Basically it says that the moon emits (really reflects but that’s immaterial) F photons per second per square meter, and while you can concentrate that into a very small area, all of those photons will not be enough to raise th…
The thing is, radiative processes (like light) can’t store energy. To do that, you need some kind of engine, either electrical or mechanical.
Of course you could put a solar panel connected to a battery in moonlight for a few months and build up enough stored energy to power a laser for long enough to fry something. But that’s not “burning something with moonlight using a magnifying glass.”
Re: Can you use a magnifying glass and moonlight to light a fire? (2016)
#126Earlier 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…
Well I think the thermodynamic analysis is useful in understanding things, that was my point. I’m not quite understanding the scenario you describe - are you asking how it is possible that you can’t heat up the target more than the origin? If so then take an example where the sun is a single point at 5000 degrees. It is easy to see that the most you could do with this point sun is heat up one point on your object to the same temperature, no more. Now what if there were two point suns next to each other? The discussion on xkcd about optics is saying you can’t superimpose the points on each other on the target under any circumstances, even with two separate lenses at the right angle. This doesn’t seem like it should be impossible though...
What about two single photon sources, can’t they be pointed at exactly the same spot? Maybe the explanation here is that the target electron cannot interact with 2 photons at the same time, so you can’t ‘double heat’ a single particle. Or maybe that you can’t precisely target a single particle without decreasing the entropy of a closed system, which is impossible.
Re: Can you use a magnifying glass and moonlight to light a fire? (2016)
#127Earlier 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…
So you have a blackbody source, and a target.
Initially it's in a steady state where the target is colder.
You increase the number of photons hitting the target. The target gets hotter. All good so far.
But we have to look at how you increased that number. You need to either move the source and target closer together, or you have to use lenses to simulate the same thing.
You can keep hitting the target with photons from more and more directions, but no system of mirrors or lenses can increase the apparent brightness of the source.
As you approach the point where the source fills the entire sky, the target will approach the same temperature.
And then you can't increase the number of photons any more. Not without using a hotter source.
Re: Can you use a magnifying glass and moonlight to light a fire? (2016)
#128Earlier quoted context omitted.
> But you're right, the light from the moon is reflected sun light, plenty hot to start a fire. That's my impression as well. A blackbody has albedo 0. The moon has an albedo of around 0.12. While I suspect you can't start a fire from moonlight in practice, I don't think the arguments in this article are correct.
While I suspect you can't start a fire from moonlight in practice, I don't think the arguments in this article are correct. I guess if you're reading that as some kind of absolutely logical argument. I read stuff like that as an abstraction which just kinda works in the messy real world. Pretty much like how the typical explanation for how a wing works turns out to be an over-simplification. It only partly works that…
It’s presented as an absolute logical argument of the form “you can’t use a lens to make anything hotter than the average surface of the moon.”
Re: Can you use a magnifying glass and moonlight to light a fire? (2016)
#129Here'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 brick pile melts.
Short of letting the brick pile melt itself, imagine tapping into the excess heat and using it to power an electric motor for a useful purpose.
Re: Can you use a magnifying glass and moonlight to light a fire? (2016)
#130So thinking about this some more, the argument here is that if you use “just” a magnifying glass of arbitrary size and shape you can’t do this. I can buy that based on the arguments presented. Basically it says that the moon emits (really reflects but that’s immaterial) F photons per second per square meter, and while you can concentrate that into a very small area, all of those photons will not be enough to raise th…
The thing is, radiative processes (like light) can’t store energy. To do that, you need some kind of engine, either electrical or mechanical. Of course you could put a solar panel connected to a battery in moonlight for a few months and build up enough stored energy to power a laser for long enough to fry something. But that’s not “burning something with moonlight using a magnifying glass.”
Right. That’s what I am saying. Using the original setup of the problem, you cannot use a device to store energy.