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)

#91

Post by analog31 » Sat, Dec 22, 2018, 1:25 AM UTC

Analemma_ wrote: post #79

Oh god, can we please get a real physicist in here? This entire thread is a mess of computer programmers “well actually”ing other computer programmers and everyone being wrong.

I'm a real physicist. But I've learned to avoid these debates because I'm not a good enough debater to avoid getting tied up in logical knots.

My version of the argument involves turning it into a perpetual motion machine.

On the other hand, this rule is incredibly useful in modeling energy transfer through optical systems, seeing what things are possible and what things aren't without having to make detailed calculations. It's a real world rule with useful applications.

Instead, I'll just spout nonsense about programming. ;-)

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

#92

Post by starbeast » Sat, Dec 22, 2018, 1:27 AM UTC

baddox wrote: post #35

nealabq wrote: post #27

Earlier quoted context omitted.

If the light is being radiated from a black body surface, then you cannot make something hotter than that surface. But you're right, the light from the moon is reflected sun light, plenty hot to start a fire. The moon also radiates like a black body, but virtually all that light has longer wavelengths than the reflected visible light. You could start a fire from the light of a single star if you had a big enough lens…

> 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

Ivanpah may be able to, I wonder if we can borrow it at night for an experiment - https://en.wikipedia.org/wiki/Ivanpah_Solar_Power_Facility

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

#93

Post by amelius » Sat, Dec 22, 2018, 1:34 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.

> Except it's not "heat energy", it's electromagnetic energy that's getting transferred.

Thermal radiation == electromagnetic radiation.

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

#94

Post by DenisM » Sat, Dec 22, 2018, 1:38 AM UTC

08-15 wrote: post #89

08-15 wrote: post #43

Earlier quoted context omitted.

> 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. (…

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 produce higher temperature at the target than it was at source.

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

#95

Post by gnopgnip » Sat, Dec 22, 2018, 1:47 AM UTC

btilly wrote: post #58

nickparker wrote: post #34

Earlier quoted context omitted.

You've found the right question to ask. Your mirror in sunlight works because the reflectivity or albedo of the mirror is very high relative to whatever target you're lighting on fire. In a magical closed system where radiative heat transfer was the only factor, objects of differing reflectivity would eventually reach temperature equilibrium through black body radiation. We aren't interested in closed systems though.…

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.

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

#96

Post by DenisM » Sat, Dec 22, 2018, 1:48 AM UTC

pierrebai wrote: post #37

wcoenen wrote: post #23

Earlier quoted context omitted.

> 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. There's a difference. By going through solar panels you would increase entropy because the conversion of sunlight to electricity can't be 100% efficient[1]. And if you allow the solar panels to heat up too much because of th…

The region of higher temperature is much smaller. Entropy is not a pure measure of energy or randomness, but a measure of that within a volume . You'd have smaller entropy in a small volume, but higher around it because you would have diverted the ray that would have hit outside the concentrated region. Were is this reasoning wrong?

Yeah this is all rather confusing...

This boils down to a moderately heated BB receiving an large stream of moderately powered photons and either rejecting them or first absorbing and then radiating them away at the same pace without changing its own temperature, regardless of how many photons are coming in.

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

#97

Post by nkurz » Sat, Dec 22, 2018, 1:57 AM UTC

CydeWeys wrote: post #86

Analemma_ wrote: post #79

Oh god, can we please get a real physicist in here? This entire thread is a mess of computer programmers “well actually”ing other computer programmers and everyone being wrong.

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). A lot of the counter-arguments/speculation here in the comments is wrong. Reading this is is equivalent to reading a thread on a physics forum where with people arguing about an article saying that O(n*lgn) is really the best possible runt…

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. Where does your certainty come from?

With that out of the way, could you give some links to well known arguments that you refer to? Specifically, I feel certain that one can start a fire with sunlight reflected from a room temperature mirror, and don't understand the difference between a mirror and the moon within Munroe's argument.

His conclusion might be correct (in practice, you may not be able to concentrate moonlight enough to start a fire) but I don't think the details of his argument can be. I currently don't believe that the temperature of the reflecting surface can be the limiting factor, and I think this is central his argument.

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

#98

Post by sandworm101 » Sat, Dec 22, 2018, 2:01 AM UTC

taneq wrote: post #84

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…

> 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 optics could probably collect and point enough moonlight to light a match. If we call fiber a sort of flexible lens, then lenses can start fires.

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

#99

Post by InclinedPlane » Sat, Dec 22, 2018, 2:01 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…

Yup, lunar landings didn't last long enough to experience the full range of extreme lunar conditions.

This is one of the reasons why lunar bases will actually be more difficult than people think. Dealing with extreme environmental conditions is much easier when they are stable, then you can design around them and deal with them. Dealing with conditions that cycle from one extreme to another continuously over extended periods of time is much more challenging. One point of proof of that is the longevity of the Martian and lunar rovers and landers. Many landers and rovers on Mars have lasted for years, some have lasted over a decade. No lunar rover has been able to maintain roving operations longer than a few months. This despite the fact that lunar rovers are in near real-time contact with Earth continuously. The heat/cold cycles and hyper-abrasive clingy dust make the lunar environment particularly harsh on equipment.

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

#100

Post by Gatsky » Sat, Dec 22, 2018, 2:04 AM 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…

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.