The Naked Scientists

The Naked Scientists Forum

Author Topic: Can you start fires with moonlight?  (Read 20366 times)

Offline Luke

  • First timers
  • *
  • Posts: 1
    • View Profile
Can you start fires with moonlight?
« on: 24/10/2011 00:01:02 »
Luke McNeill  asked the Naked Scientists:
   
Dear Nakeds

Everybody knows you can use a magnifying glass to focus sunlight to burn things, and the larger the lens, the more intense the heat. How large of a lens would you need to use to focus light from the full moon to burn paper?

Thanks,
Luke McNeill
Arlington, MA, USA

What do you think?
« Last Edit: 24/10/2011 00:01:02 by _system »


 

Offline Airthumbs

  • Hero Member
  • *****
  • Posts: 958
  • Personal Text
    • View Profile
Can you start fires with moonlight?
« Reply #1 on: 24/10/2011 00:18:21 »
I know people peer at the moon through pretty powerfull telescopes and don't seem to get any damage to their retina so maybe you would need an absolutely huge magnifying glass  :o
 

Offline Soul Surfer

  • Neilep Level Member
  • ******
  • Posts: 3345
  • keep banging the rocks together
    • View Profile
    • ian kimber's web workspace
Can you start fires with moonlight?
« Reply #2 on: 24/10/2011 08:53:51 »
No I don't think that you could, because of the limitations of conventional geometrical optics, because it is not possible to concentrate an image of an object to a higher radiation intensity than the object itself.  The sunlit surface of the moon gets well above the boiling point of water but not as high as the ignition temperature of  tinder so you might be able to make a cup of tea but not start a fire.

By the way looking at the full moon at low magnification in a medium sized telescope is quite painful and not recommended.   I speak fro experience with my own 8 inch (200mm ) Schmidt Cassegrain Reflector.
« Last Edit: 24/10/2011 08:56:52 by Soul Surfer »
 

Offline syhprum

  • Neilep Level Member
  • ******
  • Posts: 3818
  • Thanked: 19 times
    • View Profile
Can you start fires with moonlight?
« Reply #3 on: 25/10/2011 06:41:06 »
The subject of whether the concentrated spot of light from an optical system could exceed the temperature of the source of radiation was discussed at length recently on this forum.
The concensus of opinion was that it could which rather surprised me, if this is so it must apply to moonlight as well as sunlight although I cannot visualise an optical system that would do it.

SS "because it is not possible to concentrate an image of an object to a higher radiation intensity than the object itself" 
« Last Edit: 25/10/2011 06:42:50 by syhprum »
 

Offline Geezer

  • Neilep Level Member
  • ******
  • Posts: 8328
  • "Vive la résistance!"
    • View Profile
Can you start fires with moonlight?
« Reply #4 on: 25/10/2011 08:06:59 »
because it is not possible to concentrate an image of an object to a higher radiation intensity than the object itself.

That's a great point. It sounds very likely to me, but I've really no idea why! Is there a theorem that explains why that is the case?
 

Offline JP

  • Neilep Level Member
  • ******
  • Posts: 3366
  • Thanked: 2 times
    • View Profile
Can you start fires with moonlight?
« Reply #5 on: 25/10/2011 17:13:32 »
I'm with you Geezer.  It sounds likely, but I don't understand why it has to be the cast.  Initially, it would seem that the second law of thermodynamics requires it.  After all if you form an image of a single point on a black body, that point will form a slightly larger blob (assuming it's diffraction limited), which means that the temperature is lower than that of the point source.  If it somehow was a higher temperature, then it would radiate back to the source point, heating it up, and you could use it to create a perpetual motion machine.

Where I get confused is what happens with an extended source.  In that case you have many points being imaged, and although each point-to-point image has to lose temperature, if your magnification is less than one (meaning your image is smaller than the object), then you're concentrating this energy, and I can't come up with a simple argument of why you can't increase the temperature this way.  This is a lot like compressing a gas to raise it's temperature, although in the case of a gas, the compression requires mechanical work.  In the case of light, it just requires a lens.  I suspect there is a good argument of why an imaging system can't do this, even with magnification, but I can't come up with it off the top of my head.

I also recall seeing an article by Prof. Roland Winston (U. of Chicago), which I cited in the previous debate over this.  He claimed that by using a particular nonimaging system, he was able to focus sunlight to heat an object up to hotter than the temperature of the sun.
 

Offline Pmb

  • Neilep Level Member
  • ******
  • Posts: 1838
  • Physicist
    • View Profile
    • New England Science Constortium
Can you start fires with moonlight?
« Reply #6 on: 25/10/2011 21:34:17 »
I'm with you Geezer.  It sounds likely, but I don't understand why it has to be the cast.  Initially, it would seem that the second law of thermodynamics requires it.  After all if you form an image of a single point on a black body, that point will form a slightly larger blob (assuming it's diffraction limited), which means that the temperature is lower than that of the point source.  If it somehow was a higher temperature, then it would radiate back to the source point, heating it up, and you could use it to create a perpetual motion machine.

Where I get confused is what happens with an extended source.  In that case you have many points being imaged, and although each point-to-point image has to lose temperature, if your magnification is less than one (meaning your image is smaller than the object), then you're concentrating this energy, and I can't come up with a simple argument of why you can't increase the temperature this way.  This is a lot like compressing a gas to raise it's temperature, although in the case of a gas, the compression requires mechanical work.  In the case of light, it just requires a lens.  I suspect there is a good argument of why an imaging system can't do this, even with magnification, but I can't come up with it off the top of my head.

I also recall seeing an article by Prof. Roland Winston (U. of Chicago), which I cited in the previous debate over this.  He claimed that by using a particular nonimaging system, he was able to focus sunlight to heat an object up to hotter than the temperature of the sun.
If you store the energy and release it all at once with lenses toward the target.

Best wishes

Pete
 

Offline JP

  • Neilep Level Member
  • ******
  • Posts: 3366
  • Thanked: 2 times
    • View Profile
Can you start fires with moonlight?
« Reply #7 on: 25/10/2011 21:56:41 »
Well if you store it, it's obviously going to be able to make something hotter than the moon's surface.  The question I have is about heating a black body using only light emitted from another black body along with optical components (lenses, mirrors, etc.) 
 

Offline CliffordK

  • Neilep Level Member
  • ******
  • Posts: 6321
  • Thanked: 3 times
  • Site Moderator
    • View Profile
Can you start fires with moonlight?
« Reply #8 on: 25/10/2011 22:06:00 »
If you have a point source, such as a lightbulb, then it can not generate any more heat than the source itself, although, you could perhaps generate a method to capture and store the energy.

There are a few problems with extending that analogy to the Moon.

First of all, most of the light we see is not radiated heat, but rather reflected light.  Consider the moon as being a large mirror as a component of your solar collector.  Certainly the mirrors in your solar collector don't have to heat up as much as the target.

The other point is that the moon is not a point source, but rather a diffuse source with a diameter of about 3,400 km in diameter.  If you could capture 99% of the light reflected from the moon's surface, it would be like having a solar collector 3,400 km in diameter, reflecting that light onto a point-source, it would get hot.

The problem is that we can't collect 99% of the light reaching and being reflected from the moon.

Anyway, I'd say you would need an awfully big solar collector.  I wonder what you'd get from one of the large commercial solar arrays.
 

Offline yor_on

  • Naked Science Forum GOD!
  • *******
  • Posts: 11993
  • Thanked: 4 times
  • (Ah, yes:) *a table is always good to hide under*
    • View Profile
Can you start fires with moonlight?
« Reply #9 on: 26/10/2011 02:41:14 »
Assuming that you can concentrate/direct photons by some lens to hit a specific spot it should be possible? It's not as the photons from the moon has a 'cold energy' :) Is it?

"After all if you form an image of a single point on a black body, that point will form a slightly larger blob (assuming it's diffraction limited), which means that the temperature is lower than that of the point source.  If it somehow was a higher temperature, then it would radiate back to the source point, heating it up, and you could use it to create a perpetual motion machine." is a good argument though.
 

Offline imark70

  • First timers
  • *
  • Posts: 1
    • View Profile
Can you start fires with moonlight?
« Reply #10 on: 31/10/2011 05:44:40 »
I've heard that the answer's 'no'. The proof offered is that we can point an instrument at an object in space and tell it's temperature. That is to say, we can measure the heat the object radiates and the moon is not hot enough.
 

Offline Bored chemist

  • Neilep Level Member
  • ******
  • Posts: 8655
  • Thanked: 42 times
    • View Profile
Can you start fires with moonlight?
« Reply #11 on: 31/10/2011 07:10:10 »
You can't do it with a magnifying glass or other conventional optics. In principle it's possible using non-imaging optics.
 

Offline Geezer

  • Neilep Level Member
  • ******
  • Posts: 8328
  • "Vive la résistance!"
    • View Profile
Can you start fires with moonlight?
« Reply #12 on: 31/10/2011 07:54:53 »
You can't do it with a magnifying glass or other conventional optics.

Yes, but why is that?
 

Offline Geezer

  • Neilep Level Member
  • ******
  • Posts: 8328
  • "Vive la résistance!"
    • View Profile
Can you start fires with moonlight?
« Reply #13 on: 31/10/2011 15:58:00 »
Oh, I think I might have figured it out.

It's a matter of focus and color (wavelength). When you focus the light through an optical magnifying thingy, you create an image of the object on a surface. When it's properly focused, the colours of the object are as true as they can be for the particular optical system being used.

This applies to visible and also to invisible infrared (IR) light. The maximum intensity (temperature) of the IR occurs when it is properly focused on the surface, and assuming there is no attenuation of the IR through the optical system, the temperature of the image is the same as the temperature of the object. If it was not, the colors would look all wonky.

Unless you are using a mirror, the colors do have slightly different focal distances. During my pyromanic youth, I seem to remember that the highest temperatures could only be achieved with a magnifying glass when the image of the sun focused on the piece of wood I was igniting was a bit blurry. I never understood why, but maybe I do now.
 

Offline JP

  • Neilep Level Member
  • ******
  • Posts: 3366
  • Thanked: 2 times
    • View Profile
Can you start fires with moonlight?
« Reply #14 on: 31/10/2011 17:22:33 »
I'm not sure the spectral argument holds water, though.  It's emitted spectrum that tells you the temperature of a black body, not absorbed spectrum.  If I pumped light with a solar spectrum onto a point, then the equilibrium temperature of a black body at that point should be determined by the rate of energy flowing into it and it's surface area.

For example, if I project a magnified image of a light bulb onto a black body screen, that screen will be at a much lower temperature than the light bulb itself, since the surface area of the screen is much larger. 
 

Offline Geezer

  • Neilep Level Member
  • ******
  • Posts: 8328
  • "Vive la résistance!"
    • View Profile
Can you start fires with moonlight?
« Reply #15 on: 31/10/2011 17:42:59 »
Er well, OK then. What I obviously intended to say is that the temperature of the image is only the same as the temperature of the object when the image is focused onto a teensy weensy point.
 

Offline JP

  • Neilep Level Member
  • ******
  • Posts: 3366
  • Thanked: 2 times
    • View Profile
Can you start fires with moonlight?
« Reply #16 on: 31/10/2011 19:54:53 »
Er well, OK then. What I obviously intended to say is that the temperature of the image is only the same as the temperature of the object when the image is focused onto a teensy weensy point.

Isn't that begging the question?
 

Offline Geezer

  • Neilep Level Member
  • ******
  • Posts: 8328
  • "Vive la résistance!"
    • View Profile
Can you start fires with moonlight?
« Reply #17 on: 31/10/2011 20:01:57 »
Er well, OK then. What I obviously intended to say is that the temperature of the image is only the same as the temperature of the object when the image is focused onto a teensy weensy point.

Isn't that begging the question?

Don't think so ???

It just means that the temperature has an upper limit. As the area of the image approaches zero, the temperature of the image approaches the temperature of the object.

I think I shall name this "Geezer's Conjecture."
 

Offline JP

  • Neilep Level Member
  • ******
  • Posts: 3366
  • Thanked: 2 times
    • View Profile
Can you start fires with moonlight?
« Reply #18 on: 31/10/2011 20:22:56 »
I happen to believe in the Geezer conjecture, but I don't see how it's been proven.
 

Offline Bored chemist

  • Neilep Level Member
  • ******
  • Posts: 8655
  • Thanked: 42 times
    • View Profile
Can you start fires with moonlight?
« Reply #19 on: 31/10/2011 20:44:53 »
I can't remember whose "law" it is, but you can't get the thing you are heating (hereafter called the ant) hotter than the thing heating it Hereafter called the sun).
The problem is, in essence, that you need a really big magnifying glass, but that will have a long focal length (because there's a limit to the radius of curvature) so the image will be big so the power density will be small so the rise in temperature will be small.

However, with non-imaging optics this doesn't apply. The conceptually easiest one is just a silvered funnel with the wide end pointed at the moon. The light that goes in the big end has to come out of the small end . If the thing is big enough the output power density can be made (practically) arbitrarily large so the temperature rise can be big.
 

Offline CliffordK

  • Neilep Level Member
  • ******
  • Posts: 6321
  • Thanked: 3 times
  • Site Moderator
    • View Profile
Can you start fires with moonlight?
« Reply #20 on: 31/10/2011 21:37:30 »
Keep in mind that the majority of the light we see from the moon is reflected light, and not emitted light from the intrinsic heat of the moon (we don't see the IR bands).

You should be able to start a fire using the sun and a mirror, despite the mirror not getting red hot.



Likewise, if the moon is acting at the mirror, one should still be able to capture the sunlight and start the fire.

I suppose the task gets far more difficult if one replaces the mirror with a white wall which acts as a diffuser.  Still, it should be possible to capture the sunlight reflected by a white wall.

I would think you could get some empirical results from a large solar collector. 



This one is actually owned by the University of California, Davis, so perhaps they would be willing to run an experiment.  You would have to find a way to collect night-time moonlight without contaminating the experiment with residual daytime heat.
[Oops, never mind, apparently someone had the bright idea to tear down that one.  There are other ones though.  Perhaps the Spanish PS20]

I'm not sure about the limitations of refraction.  There is a limit to the precise focus that a telescope uses.  However, in this case, one would be perfectly happy with multiple lenses, or minor perturbations in the focus.  But, still, when considered a large collector (1 sq km?) it would be much easier to make it with mirrors.
« Last Edit: 31/10/2011 21:48:30 by CliffordK »
 

Offline Geezer

  • Neilep Level Member
  • ******
  • Posts: 8328
  • "Vive la résistance!"
    • View Profile
Can you start fires with moonlight?
« Reply #21 on: 31/10/2011 23:30:33 »
Maybe it's just because it's Halloween (here anyway) but it strikes me there's something very dodgy about the idea that you can ever concentrate the energy to make something hotter than the source. On the other hand, as long as you don't try to take more energy out than went in, why not?

 

Offline CliffordK

  • Neilep Level Member
  • ******
  • Posts: 6321
  • Thanked: 3 times
  • Site Moderator
    • View Profile
Can you start fires with moonlight?
« Reply #22 on: 31/10/2011 23:45:54 »
In this case, the source of the REFLECTED light is the sun, and not the moon.

Even so, you would have a diffuse source (at a great distance) that you're concentrating to a single point. 
 

Offline Geezer

  • Neilep Level Member
  • ******
  • Posts: 8328
  • "Vive la résistance!"
    • View Profile
Can you start fires with moonlight?
« Reply #23 on: 01/11/2011 00:10:40 »
In this case, the source of the REFLECTED light is the sun, and not the moon.

Yes, but's really no such thing as "reflected". Solar energy was absorbed by the Moon and energy was emitted by the Moon.
 

Offline Bored chemist

  • Neilep Level Member
  • ******
  • Posts: 8655
  • Thanked: 42 times
    • View Profile
Can you start fires with moonlight?
« Reply #24 on: 01/11/2011 06:59:23 »
In this case, the source of the REFLECTED light is the sun, and not the moon.

Yes, but's really no such thing as "reflected". Solar energy was absorbed by the Moon and energy was emitted by the Moon.

The same is true for light reflected by a mirror.
The important difference is that the moon is not a specular reflector. (Though, if it were, the fact that it;s not flat would make things more difficult.)
 

The Naked Scientists Forum

Can you start fires with moonlight?
« Reply #24 on: 01/11/2011 06:59:23 »

 

SMF 2.0.10 | SMF © 2015, Simple Machines
SMFAds for Free Forums
 
Login
Login with username, password and session length