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)

#201

Post by skolos » Sat, Dec 22, 2018, 9:17 AM UTC

empiricus wrote: post #198

Reading the comments I found something I don'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...

Black body photons are coming at random times. Laser photons are coherent, so are timed. Think of swing - if you try to push it randomly you'll get it swing as far as hardest push. But if you push periodically, you can swing it very far with small pushes.

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

#202

Post by empiricus » Sat, Dec 22, 2018, 9:29 AM UTC

skolos wrote: post #201

empiricus wrote: post #198

Reading the comments I found something I don'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...

Black body photons are coming at random times. Laser photons are coherent, so are timed. Think of swing - if you try to push it randomly you'll get it swing as far as hardest push. But if you push periodically, you can swing it very far with small pushes.

Thanks. So the laser is really special; because it is coherent it can be absorbed and transformed into heat without limit.
For random photons, no matter the light intensity, there is a limit temperature of the receiver, which depends on the photon spectrum...
From this point of view (energy transfer) are there more than these two kinds of light?

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

#203

Post by captainsham » Sat, Dec 22, 2018, 9:31 AM UTC

You can, if you remember "the scientific principles of the convergence and refraction of light."

"The scientific principles of the convergence and refraction of light are very confusing, and quite frankly I can't make head or tail of them, even when my friend Dr. Lorenz explains them to me. But they made perfect sense to Violet."

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 "The Wide Window" by Lemony Snicket.

It's possible that this book is a work of fiction.

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

#204

Post by MRD85 » Sat, Dec 22, 2018, 9:31 AM UTC

skolos wrote: post #201

empiricus wrote: post #198

Reading the comments I found something I don'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...

Black body photons are coming at random times. Laser photons are coherent, so are timed. Think of swing - if you try to push it randomly you'll get it swing as far as hardest push. But if you push periodically, you can swing it very far with small pushes.

But won't two hard pushes in a row send you flying? I would have presumed you could find the mean and standard deviation of both the energy of the pushes and the frequency of the pushes and you could build a model to find the expected height.

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

#205

Post by markbaikal » Sat, Dec 22, 2018, 9:37 AM UTC

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/ultraviolet). This is called selective coating.

https://en.m.wikipedia.org/wiki/Solar_thermal_collector

Combined with the lens, this might work.

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

#206

Post by peterburkimsher » Sat, Dec 22, 2018, 9:45 AM UTC

Is it possible to light a fire using sunlight during an eclipse?

If not, at what percent totality does it become impossible?

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

#207

Post by LeifCarrotson » Sat, Dec 22, 2018, 11:13 AM UTC

nickparker wrote: post #34

parliament32 wrote: post #12

> You can't use lenses and mirrors to make something hotter than the surface of the light source itself. 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't the mirror stay cool? Isn't the moon just a (poor) mirror for the sun's light?

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.…

> black body radiation is all it's got

No, reflection and black-body emission are different. The moon primarily produces the former.

Moonlight is not the result of the moon glowing incandescent.

The surface is very cold, its emissions as a black-body radiator are in the far infrared.

Moonlight is white light with a color temperature of several thousand Kelvin. This is reflected sunlight and has nothing to do with black-body radiators.

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

#208

Post by Dylan16807 » Sat, Dec 22, 2018, 11:14 AM UTC

AlexCoventry wrote: post #200

Dylan16807 wrote: post #193

Earlier quoted context omitted.

You get the highest temperature by reflecting moonlight from every angle. For a single lens you just want something really big. Make it take up 90+% of the sky from your target. For temperature, the no-calculation way is to measure a rock on the moon (article says 100C) and use that number for how hot you should be able to get. The calculation way goes as follows: Near earth you get 1400 watts per square meter of sun…

> You get the highest temperature by reflecting moonlight from every angle. I see the thermodynamic principle at work, here, but I don't really understand how it's operating at a mechanistic level. Is it possible to demonstrate that assertion optically?

Take your target and trace a ray in every direction away from every point on its surface. The more of these that hit the energy source, the more energy you're getting. And 100% is obviously the best you can do.

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

#209

Post by robbiep » Sat, Dec 22, 2018, 11:16 AM UTC

skolos wrote: post #197

skolos wrote: post #145

Interesting article. Right in many places. Wrong (possibly) in main conclusion. 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 radiati…

Looking through comments. There is still lots misunderstanding. So let's get deeper into the topic. In particular - how stuff gets heated by light and what light "temperature" is. Thermodynamics arguments are always good but you really need to understand that most of them describe closed systems under equilibrium. Real world is messier and if your understanding of thermodynamics is shaky its easy to get to wrong conc…

I am not an expert but on your oversimplification on the ‘High energy = higher wavelength’

Whilst this is true, it is also a bit irrelevant. Gamma rays are higher energy than visible light but you don’t need to focus gamma radiation (should that be easily possible) to start a fire, you can do it with visible light; or infrared light (which is lower wavelength again) at a high enough qanta

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

#210

Post by pontifier » Sat, Dec 22, 2018, 11:38 AM UTC

pontifier wrote: post #156

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. It struck me as very similar to the setup in this video. https://youtu.be/awADEuv5vWY 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. It seems plausib…

I think I finally realised what rubs me the wrong way about these xkcd musings, and the related discussions.

[Start rant]

They take the situation to its absurd conclusion, then quit with a full finality that people take as truth. Further absurd conclusions are ignored, and their word is law.

Some more absurd arguments for why you will be able to light a fire with a magnifier and moonlight are as follows:

1: use a pre-magnifier to create a spot on the moon with the same temperature as the sun, then magnify the light from that spot to start your fire.

2: wait a long time... eventually a meteor will hit the moon creating a spot bright enough to focus.

3: wait even longer... Eventually the random mollecular collisions between the wood and the (presumably oxegen rich) air around it will convert it into carbon dioxide and water while serendipitous individual high energy blackbody photons help break it down.

4... insert absurd^4 answer that brings in tunneling effects, or moving mirrors, or some other "impossible" reason that is only impossible because they didn't think of it for you. [End rant]