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)

#111

Post by DenisM » Sat, Dec 22, 2018, 2:25 AM UTC

Misdicorl wrote: post #105

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 entropy argument is a red herring for your system (and wrong). The difference is that you would have to store your energy from the solar panel for time t1 and then expected it in time t2 much less than t1 to achieve your desired effect. The system in question doesn't allow storing the energy for later quicker dispersal

No, I do not store any energy. My solar panels outputs are wired directly to the input of an arc welder. Powered from the sun light (surface temperature 6,000C) this system can easily give me an arc of 10,000 C.

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

#112

Post by tbabb » Sat, Dec 22, 2018, 2:36 AM UTC

sandworm101 wrote: post #98

taneq wrote: post #84

Earlier quoted context omitted.

> 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 I don't understand this bit. I don't think there's any power law involved in reflection, if something has an albedo of 0.12 then it just reflects 0.12 times the incident radiation, doesn't it? I think I agr…

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 same situation ("moon visible in all directions") could be created with a few lenses and mirrors.

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

#113

Post by aasasd » Sat, Dec 22, 2018, 2:37 AM UTC

detaro wrote: post #103

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,…

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!

\($ ∇ $ )/

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

#114

Post by btilly » Sat, Dec 22, 2018, 2:37 AM UTC

gnopgnip wrote: post #95

btilly wrote: post #58

Earlier 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…

As a comparison the moon and sun are roughly the same size in the sky from earth. The sun puts out about 1000 watts per square meter on a clear day at noon in summer, the moon puts out about 0.0025 watts per square meter during full moon on a clear day, 1/400000 as much.

Your figure for the energy from the Moon seems overstated. https://education.seattlepi.com/moonlight-strong-enough-powe... quotes a ratio of 2.3 million to one.

However the key point is this. The Moon is in reality 400 times smaller than the Sun. Which means that an optimally placed lens can actually make the image of the Moon 400 times smaller, for 160,000 times as little area. If the lens is big enough that this target area is mostly losing heat through black body radiation, again it should wind up over half the temperature of the surface of the Sun. Which is more than hot enough to start a fire.

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

#115

Post by sandworm101 » Sat, Dec 22, 2018, 2:46 AM UTC

tbabb wrote: post #112

sandworm101 wrote: post #98

Earlier 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…

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).

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

#116

Post by gowld » Sat, Dec 22, 2018, 2:47 AM UTC

jarfil wrote: post #68

caf wrote: post #62

Earlier quoted context omitted.

What the thermodynamic principle says you can't do is to take Q units of heat energy from one body and transfer it all to a higher temperature body. The solar panel system is different - it's a heat engine that takes Q units of heat energy from one body, transfers a portion of it Q₁ to a higher temperature body and transfers the remainder of it (Q - Q₁) to a lower temperature body (the immediate surroundings). That's…

> you can't do is to take Q units of heat energy from one body and transfer it all to a higher temperature body Except it's not "heat energy", it's electromagnetic energy that's getting transferred. If the target object is smaller than the source object, you can have an amount of energy that will increase the target's heat above that of the source.

It's impossible to aim the source at a smaller target because the source dissipates in all directions, unless you make something like a laser.

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

#117

Post by wrycoder » Sat, Dec 22, 2018, 2:54 AM UTC

adammunich wrote: post #24

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.

You are describing a parabolic concentrator.

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

#118

Post by petermcneeley » Sat, Dec 22, 2018, 3:05 AM UTC

zeristor wrote: post #38

So what about non-linear optics and optical frequency doubling? https://en.wikipedia.org/wiki/Second-harmonic_generation It was just an option on my MSc in LASERs but I thought it was cool (with the potential to be very warm). Although my frequency has been halved and I've been working in software for decades

I thought of that as well. Once you have a different frequency you can also do more bizarre fun things. http://optics.org/news/7/1/11

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

#119

Post by Dylan16807 » Sat, Dec 22, 2018, 3:10 AM UTC

IgorPartola wrote: post #53

I don’t buy the argument that you can’t concentrate two beams on the same spot. Sure you might not be able to do that with one lens, but concentrating on a small area is as good an approximation. But if you require that it really be a point, why can’t I do that with N lenses and mirrors that are fully reversible but all align to aim at the same point from different angles?

The argument is that you can't concentrate two beams to hit the same spot from the same direction.

So yes, you can have a bunch of lenses each focusing from a different direction.

And if you do a good job of aligning them, each lens will look as bright as the sun/moon from the target.

But that's your limit.

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

#120

Post by nkurz » Sat, Dec 22, 2018, 3:12 AM UTC

mrbrowning wrote: post #108

nkurz wrote: post #97

Earlier quoted context omitted.

The linked article is correct and the arguments are well known and accepted in the physics community and have been for a long time (much longer than Randall Munroe has been alive). It's interesting what bothers different people. While many of the statements in this thread are probably wrong, not many of them bother me. But I find the lack-of-self-doubt and appeal-to-authority in your message to be genuinely offensive…

"Appeal to authority" is a fallacious rhetorical tactic only in an extraordinarily rigorous epistemological context, in which all truths need to be established from first principles. In practice, especially given the information asymmetry between the people that have devoted rigorous study to a thing and the people that think they know about it because making computers do things means they're smart (which is an intel…

I agree on most of your points. There is nothing inherently wrong with an appeal-to-authority in this case. If any substantial number of "reliable" physicists have looked at this problem, and if they unanimously agree that there is no way to concentrate light reflected from an object to produce a temperature greater than that object, then my intuition is almost definitely wrong and their conclusion is almost definitely right.

The nice part about physics though (like Munroe, I was also an undergraduate physics major) is that in simple cases like this, a suitable expert can usually defend their position with an argument comprehensible to a nonspecialist outsider. My doubt in this case is not that the experts are wrong, but that the experts haven't actually looked at the details of Munroe's argument and stamped it as "approved".

I think the part I find "offensive" is that CydeWeys is not claiming to be an expert himself, but is claiming to have certainty in what the experts believe. I don't know exactly why I find this offensive, but I do. And yes, this may be a problem with me, and not with CydeWeys' argument. I would not be offended in the same way by someone claiming "I am an expert and I approve this argument". Still, my question to him is genuine: what gives him this certainty?