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

Interesting article. Right in many places. Wrong (possibly) in main conclusion.<p>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 radiation&#x27;s temperature.<p>Also the argument about impossibility of concentrating light into a dot is correct (although even if it were possible we still would not be able to get higher temperature - light would not be energetic enough for that). The important part is - we could concentrate light into a dot only if it consist of parallel rays - i.e. only for an object that is infinitely far away.<p>Moon surface temperature argument is incorrect. A body at 100 degrees Celsius does not radiate in visible spectrum, so the light we see is not produced by Moon&#x27;s temperature. It is reflected Sun light. So Moon&#x27;s temperature doesn&#x27;t matter. Moon surface does absorbs some light, changing spectral composition from about 5.7kK (Sun&#x27;s surface temperature) to about 4kK. So we should consider moon to be a part of optics not emitter.<p>Hence the question is now - can we concentrate moon light enough so that intensity at the concentration point is higher than thermal loss into environment (only then we will be able to raise temperature in the concentration area enough for combustion - remember that light is &quot;hot&quot; enough for this)? I don&#x27;t have answer for that - need to do calculations. What can be a deal breaker? Remember that Moon is much closer than Sun, so rays come to us even less parallel, so the area into which we can concentrate light reflected from the Moon is even larger than the Sun&#x27;s, so together with lower intensity of light from Moon we might have trouble achieving the necessary intensity for combustion. However big enough lens probably will work.<p>And yes - I&#x27;m a physicist by training.

&gt; You can&#x27;t use lenses and mirrors to make something hotter than the surface of the light source itself.<p>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&#x27;t the mirror stay cool? Isn&#x27;t the moon just a (poor) mirror for the sun&#x27;s light?

Wow... separately I had no idea the surface of the moon reaches (and goes above) 100°C... that&#x27;s <i>hot</i>! Literally boiling hot. Turns out at night it goes down to almost –200°C. That&#x27;s <i>insane</i>.<p>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&#x27;t conduct heat well (all dust?), of course there&#x27;s no atomsphere to conduct heat, and that their boots were extremely well-insulated.

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.<p>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 the dispersion is dictated by that of the source. That is you can&#x27;t make the target denser then source, hence the temperature must be less.<p>EDIT: Several commenters submitted that lenses are reversible while solar panels are not, and this makes all the difference. My retort is that I can make non-reversible lenses by covering them in a thin layer of dust. Since the lense system is now non-reversible can I use these sub-par lenses to create higher temperature than I could with clear lenses?

So 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 the temperature (that is input enough energy into the system) to a sufficiently high level. This is partially because you can’t make a small enough point with a single lens, and partially because there just aren’t enough photons. The sun doesn’t care because it has such a high flux that the concentration ends up being high enough for the area.<p>The area argument is more important here because the lens cannot increase the number of photons per second, but it can decrease the area. If F = N &#x2F; (t * A) where N is the number of photons, t is time, and A is area, the lens can change the area, but not to 0. And if you need a sufficiently high F to get to the right temperature, the only way to get there with limited N is to bring A sufficiently close to 0.<p>If you have multiple magnifying glasses and mirrors I am fairly certain that you can. That is the equivalent of using a set of solar panels that power a laser. But that was not what was postulated in the original thought experiment, so it does not apply.<p>I am still fuzzy on the thermodynamic argument, but I was never good at intuiting thermodynamics. The argument presented is that if you have one body at 100 degrees C, and you put another body next to it, you cannot make the second body hotter than the first. That makes sense. But if the first body is constantly generating and transferring heat to the second with at most 100 degrees C temperature, <i>and</i> the second body has some way to store heat energy, then it is possible to heat a local area of the second body to higher than 100 degrees C. The storage of energy here is what I think counts.

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.

Physicists here.<p>He&#x27;s trying to explain everything in terms of a simple blackbody in thermal equilibrium, peacefully radiating its energy away only via thermal photons. That&#x27;s not the reality of the radiation from the sun or moon. Solar physics is an entire branch of physics, and such simple toy models are not even wrong.<p>Sun doesn&#x27;t just radiate away its existing energy via thermal photons.<p>First, it keeps burning its fuel via a series of nuclear reactions, which by the way keeps pumping energy into the system, essentially acting like a battery (so there&#x27;s no perpetual motion here).<p>Second, sun emits photons that are much more energetic than the thermal photons from the surface. Some of the radiation is not thermal, and comes directly from different types of nuclear reactions (which provides signatures regarding the kind of reactions happening in the sun) and various other processes.

Discussed at the time: <a href="https:&#x2F;&#x2F;news.ycombinator.com&#x2F;item?id=11211454" rel="nofollow">https:&#x2F;&#x2F;news.ycombinator.com&#x2F;item?id=11211454</a>

So what about non-linear optics and optical frequency doubling?<p><a href="https:&#x2F;&#x2F;en.wikipedia.org&#x2F;wiki&#x2F;Second-harmonic_generation" rel="nofollow">https:&#x2F;&#x2F;en.wikipedia.org&#x2F;wiki&#x2F;Second-harmonic_generation</a><p>It was just an option on my MSc in LASERs but I thought it was cool (with the potential to be very warm).<p>Although my frequency has been halved and I&#x27;ve been working in software for decades

I don’t buy the thermodynamic argument. Here’s a version I would believe: if you have a gadget that, exposed only to the sun and to empty space, heats some target hotter than the sun, then that gadget must <i>not</i> work if you take away the empty space part. This is because your gadget could be used to drive a heat engine, which is impossible without a temperature difference, and the sun is more or less a blackbody emitter. Lenses and mirrors aren’t magically taking advantage of the cold parts of the sky, so there you go.<p>But the moon is not blackbody, and I think the whole argument falls apart. Here’s a thought experiment: go stand on the moon, and assume the moon is made of rock that diffusely reflects, say, half of the indicent 500nm light. Stand somewhere that’s in shadow, so you can’t see the sun. Wrap a piece of paper and some air in perfectly insulating, perfectly reflecting material, except that the material lets 100% of 499-501nm light through, but only on the moon side. The target will be in a bath of 499-501nm light at 1&#x2F;2 the intensity (energy density per unit volume) of the sun, which is far more than half the temperature of the sun. It’ll catch fire after a while.<p>Now do the same experiment on the Earth, at night, with lenses to bathe it in moonlight from all sides. Fire! So I claim that lenses+mirrors+filters can start a fire with moonlight.<p>Another interesting question: can you use a luminescent solar concentrator or other fluorescent material to pull this off without taking such egregious advantage of the spectrum of moonlight? These types of materials can violate conservation of étendue.

The article reasoning can be shortened to observation that passive optical system does not change the wavelength of photons and to trigger a fire the wavelength has to be short enough.<p>But the conclusion of the article is wrong. The surface temperature of the Moon has very little to do with the wavelength of the reflected photons.<p>Consider a surface covered with ideal tiny mirrors each pointing to random direction. Only a tiny proportion of these mirrors will reflect light from the Sun towards observer. Now consider that 90% of those mirrors are painted black reducing the reflected enrrgy flux by further factor of ten. The Moon is like that.<p>Still the reflected light has original wavelength of the light of the Sun. Collect enough of it and that triggers fire.

Well, this doesn&#x27;t look right to me. Imagine that the moon is actually a filter at the path of the sunlight.<p>Sun&#x27;s temperature is 6000 K. Moon&#x27;s surface is pretty black: it reflects only 12% of the light.<p>So, effective temperature of the Sun reflected by Moon, considering that thermal radiation is proportional to T^4, is 6000 * .12 ^ (1&#x2F;4) ~= 3500 K. That&#x27;s quite enough to light up some fire! Of course, the spectral composition of the light will be not thermal etc, but the estimate should be close enough.<p>Why doesn&#x27;t the Moon itself heat up like that? Well, the rocks on Earth don&#x27;t heat up to 6000 K either... I think, it&#x27;s partly that they are &quot;not surrounded by sun&quot;, partly that the Moon is a giant cold heatsink

I&#x27;m a bit skeptical of the conclusion and the reasoning process used to arrive there.<p>For example, the article states &quot;In other words, all a lens system can do is make every line of sight end on the surface of a light source, which is equivalent to making the light source surround the target.&quot;<p>If you forget about optics for a second, imagine that the outer surface of the sun were wrapped around a point (think of the image shown in the article). If you consider conservation of energy for the energy flux from the surface of the sun being entirely directed to a single body of matter that absorbs this heat (assume it&#x27;s a penny), the steady-state blackbody emission of the penny would have to equal the energy flux from the entire surface of the sun. I think this situation would end up making the &#x27;temperature&#x27; of the penny much higher than the surface of the sun for the same reason that the center of the sun is hotter than the surface: There is energy expended, and it comes from the fusion of light elements inside the sun, so there is no violation of entropy as stated in the article: &quot;you&#x27;d be making heat flow from a colder place to a hotter place without expending energy.&quot;

Isn&#x27;t the bigger problem that sunlight is nearly parallel and moonlight is reflected off of a spherical surface?<p>How are you violating conservation of energy if you&#x27;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?<p>If you can&#x27;t concentrate light that way then how do focusing lenses on cutting lasers function? Makes no sense.

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?

I&#x27;m not so sure about this.<p>Imagine you had a large cloud of planar mirrors, each, specifically can be aimed at any given point --even points that overlap.<p>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.

&gt; Lenses and mirrors work for free; they don&#x27;t take any energy to operate.<p>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?<p>I guess that lenses are ‘free’ only if they have no impurities, but even then, assuming solar panels are costlier than plastic lenses, I could save money.<p>Come to think of it, how is it that I haven&#x27;t seen or heard about parabolic reflectors with solar panels in the focal point? Right now I&#x27;ve found an article about parabolic troughs that are apparently used to heat old-school fluids instead: <a href="https:&#x2F;&#x2F;en.wikipedia.org&#x2F;wiki&#x2F;Parabolic_trough" rel="nofollow">https:&#x2F;&#x2F;en.wikipedia.org&#x2F;wiki&#x2F;Parabolic_trough</a>

Randall might be correct for conventional optics, but what about metamaterial lenses that break the diffraction limit? <a href="https:&#x2F;&#x2F;en.m.wikipedia.org&#x2F;wiki&#x2F;Superlens" rel="nofollow">https:&#x2F;&#x2F;en.m.wikipedia.org&#x2F;wiki&#x2F;Superlens</a>

I think They missed something. If the max temp you can get from moon light is 100c that doesn’t mean you can’t start a fire. You just need something that has a very low ignition temp. There must exist something that can start a fire at this temp.

There is a very interesting image in the article. It shows a bunch of light coming into a block and emerging as a beam. It also has a caption saying this is impossible.<p>It struck me as very similar to the setup in this video. <a href="https:&#x2F;&#x2F;youtu.be&#x2F;awADEuv5vWY" rel="nofollow">https:&#x2F;&#x2F;youtu.be&#x2F;awADEuv5vWY</a><p>At about 4 minutes in, a nearly identical setup is shown with a beam of light emerging from a block of opaque material using holographic techniques.<p>It seems plausible to me that a specially designed anti-moon hologram could allow reconstruction of the incident light from the sun, thus allowing a fire to be started without violating any law of thermodynamics.

I&#x27;m not buying this. The Moon is only a reflector, not a producer of light. The temperature of the light is that of the reflected source. Moonlight is sunlight, more or less. The fact that the moon reflects poorly is compensated by the size of the gathering lens or mirror.<p>Suppose we build a 100 foot mirror which reflects 10% of sunlight, such that the spectrum remains the same. We could still make a fire with the reflected light, if we just gather 10 times more of it with a larger lens. We could do this in the Arctic, with the mirror&#x27;s temperature at below zero; the mirror&#x27;s temperature is irrelevant.

You can, if you remember &quot;the scientific principles of the convergence and refraction of light.&quot;<p>&quot;The scientific principles of the convergence and refraction of light are very confusing, and quite frankly I can&#x27;t make head or tail of them, even when my friend Dr. Lorenz explains them to me. But they made perfect sense to Violet.&quot;<p>Violet Baudelaire goes on to use the scientific principles of the convergence and refraction of light to set fire to a piece of sail cloth using only moonlight and the lens from a spying glass in &quot;The Wide Window&quot; by Lemony Snicket.<p>It&#x27;s possible that this book is a work of fiction.

This explanation does not &quot;click.&quot; I can&#x27;t say whether it&#x27;s technically wrong; I just note that it lacks some of the features of a successful explanation. First time I&#x27;ve seen a dud from Randall.

Here&#x27;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.<p>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.<p>This in turn increases the amount of heat collected from the pile, ad infinitum or until the brick pile melts.<p>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.

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).

One has to coat the material to be set on fire with something that has low infrared absorbance (and thus low cooling through heat radiation) but high absorbance for low wavelength (blue&#x2F;ultraviolet). This is called selective coating.<p><a href="https:&#x2F;&#x2F;en.m.wikipedia.org&#x2F;wiki&#x2F;Solar_thermal_collector" rel="nofollow">https:&#x2F;&#x2F;en.m.wikipedia.org&#x2F;wiki&#x2F;Solar_thermal_collector</a><p>Combined with the lens, this might work.

I don&#x27;t really follow this argument, and I would like to.<p>I think one thing which would help me develop an intuition for it would be to see the calculation of the lens size for heating a one square-centimeter area on the earth to as high a temperature as possible by the light of the moon, and what that optimal temperature is.<p>Anyone reading for whom this is straightforward? Even a description of how to do the calculation would go a long way.

Reading the comments I found something I don&#x27;t understand. What is the difference between 1.black body photons and 2.laser photons An object will heat only up to the original temperature of the black body source in the first case, but to an arbitrary high temperature in the laser case...

I had the presumption that you could make a mega hot spot with a large enough lens back in the 90s. Fortunately usenet set me straight. It was a long and facinating journey to understand all the whys about it.

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.<p>it&#x27;s like there&#x27;s a brain bug where engineers think they can outsmart 200+ years of scientific progress with a clever arrangement of mirrors.

I&#x27;ve often wondered how big of a lens you would need to grow a plant using a light source from outside of the solar system, if it&#x27;s even possible.

Is it possible to light a fire using sunlight during an eclipse?<p>If not, at what percent totality does it become impossible?

One thing that makes me uncertain about this is the fact that the sun is generating the light from a fusion reaction, thus expending energy. In other words, the system entropy does not decrease because the fusion reaction makes up for any lost entropy by the cold-to-hot temperature flow. This is the same reason why a system consisting of a battery and a fridge would work.

I&#x27;m pretty sure if I were to go pick up a number of magnifying glasses and focus them on the same point using moonlight, the temperature at that point would increase with every additional magnifying glass.<p>Am I to accept that the additional magnifying glasses would cease increasing the temperature once the temperature matched that of the moon&#x27;s surface?

this article seems to refute some aspects of xkcd&#x27;s answer:<p><a href="https:&#x2F;&#x2F;physics.stackexchange.com&#x2F;questions&#x2F;370446&#x2F;is-randall-munroes-what-if-xkcd-correct-that-magnified-moonlight-cant-get-th&#x2F;370600#370600" rel="nofollow">https:&#x2F;&#x2F;physics.stackexchange.com&#x2F;questions&#x2F;370446&#x2F;is-randal...</a>

I love xkcd, but this is completely wrong.<p>It is well known that the spectral temperature of the moon is about 4000K. See for example : <a href="http:&#x2F;&#x2F;www.lumec.com&#x2F;newsletter&#x2F;architect_06-10&#x2F;the_sun_the_moon.html" rel="nofollow">http:&#x2F;&#x2F;www.lumec.com&#x2F;newsletter&#x2F;architect_06-10&#x2F;the_sun_the_...</a> or <a href="https:&#x2F;&#x2F;physics.stackexchange.com&#x2F;questions&#x2F;244922&#x2F;why-does-moonlight-have-a-lower-color-temperature" rel="nofollow">https:&#x2F;&#x2F;physics.stackexchange.com&#x2F;questions&#x2F;244922&#x2F;why-does-...</a> . That is the maximum temperature that you can achieve with light from the moon, no matter how concentrated. 4000 K is plenty hot enough to start a fire.

EDIT: I&#x27;m not sure about this anymore.

This is great, I always love the xkcd &quot;What if&quot; explanations. I even have the book sitting on my desk next to me.

You can with magnifying glass bigger 2.3 million times