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

what-if.xkcd.com

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

#211

Post by _0w8t »

ummonk wrote: post #184
skolos wrote: post #182

Earlier quoted context omitted.

Trying to follow your argument. Suppose we put absolutely reflective mirror on the orbit. Since its absolutely reflective its temperature will be 0 Kelvin (in theory, very low in practice - there is a reason solar bound spacecrafts are covered with reflective surfaces). So we would not be able to use the light it reflects to heat anything?

Specular reflector preserves the etendue of the light rays that fall on it. Diffuse reflector (such as the moon) does not; it becomes a new light source instead, resulting in higher etendue. In short - perfect reflector preserves etendue, but imperfect does not.

The question is to what extend the moon surface is imperfect reflector. For example, if the mirror has a hole, it will be an imperfect reflector, but you can compensate for it using a bigger lense.

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

#212

Post by _0w8t »

ummonk wrote: post #183
skolos wrote: post #177

Earlier quoted context omitted.

Leaving this comment to make sure that others don't get confused by your response. 1. As you mentioned, the moon acting as greybody absorbs part of the radiation and scatters the rest. In the second sentence you mention that the moon's temperature is enough to describe this radiation. In the third sentence you mention that you don't need moon's temperature to do that. So which is which? Correct answer - to describe r…

>Correct answer - to describe re-radiated energy we need moon's temperature, but to describe scattered we don't. We can ignore re-radiated, since it is visible light and not hot enough. We can use scattered, since it is visible and hot enough. When dealing with a gray-body, the equillibrium temperature of the body will be equal to the effective temperature of the incoming light, which will be equal to the effective t…

The moon surface is not in equilibrium as there is thermal flow towards moon interior.

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

#213

Post by xupybd »

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.

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

#214

Post by [deleted] »

nshepperd wrote: post #161
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?

The thermodynamic argument applies to black bodies, and says that you can't make an object hotter than the surface of the emitter. That's pretty uncontroversial. The more general argument based on etendue, which applies to the moon, is: you can't make the incoming light any brighter than it is on the surface of the source (which doesn't have to be the original emitter, but can be any point along the path of the light…

[deleted]

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

#215

Post by e12e »

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?

The other obvious question is: could we mold one of the moon craters to a gigantic parabola and coat it to be (much more) reflective? And have the moon shine a concentrated death ray down to earth? We'd obviously have trouble aiming - but it should make for a spectacular experiment....

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

#216

Post by skykooler »

_0w8t wrote: post #211
ummonk wrote: post #184

Earlier quoted context omitted.

Specular reflector preserves the etendue of the light rays that fall on it. Diffuse reflector (such as the moon) does not; it becomes a new light source instead, resulting in higher etendue. In short - perfect reflector preserves etendue, but imperfect does not.

The question is to what extend the moon surface is imperfect reflector. For example, if the mirror has a hole, it will be an imperfect reflector, but you can compensate for it using a bigger lense.

The moon is very imperfect; it's darker than asphalt. http://wtamu.edu/~cbaird/sq/2015/08/06/why-is-the-moon-so-br...

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

#217

Post by _0w8t »

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.

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.

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.

Still the reflected light has original wavelength of the light of the Sun. Collect enough of it and that triggers fire.

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

#218

Post by amluto »

Dylan16807 wrote: post #187
amluto wrote: post #144

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 not 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 emitte…

> The target will be in a bath of 499-501nm light at 1/2 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. Unconcentrated sunlight is slightly under 1400 watts per square meter. It's equivalent to a temperature of 122C. You can concentrate sunlight coming from the sun, because the sun only fills five millionths of the sky…

> 700 watts per square meter doesn't set things on fire. It can only heat a blackbody to 60 degrees C.

> (I'm ignoring the part about wavelength filtering because it's confusing and would only make your piece of paper heat up less.)

I’m afraid you’re ignoring the interesting bit. You say that 700 W/m^2 doesn’t set things on fire. This is not true. Sure, 700 W/m^2 applied to some target that is allowed to radiate its own blackbody light out to the sky won’t get it very hot, but that’s not what I’m suggesting. I’m suggesting that you insulate the target very well so that its blackbody emissions don’t escape, but you let in the short-wavelength moonlight. Thermodynamics requires that you also let out the short wavelength blackbody emissions, but those are negligible until the target gets very, very hot.

This effect isn’t science fiction — it’s just the greenhouse effect, amplified. Greenhouses (the glass ones and the atmospheric ones) exploit the fact that sunlight doesn’t match the Earth’s blackbody spectrum, so a filter (glass or gaseous) can allow incoming radiation in but trap most outgoing radiation.

In effect, I’m suggesting that a very good greenhouse plus some lenses could get hot enough to start a fire.

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

#219

Post by e12e »

xupybd wrote: post #213

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.

Maybe: https://en.m.wikipedia.org/wiki/Carbon_disulfide

Doesn't appear that many substances have auto ignition as low as 100c.

If you're hunting for easy to ignite stuff, it might be better to go for low flash point stuff, and strike a spark?

Eg gasoline will work in pretty cold environments with a flashpoint of - 43c.

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

#220

Post by darkmighty »

_0w8t wrote: post #211
ummonk wrote: post #184

Earlier quoted context omitted.

Specular reflector preserves the etendue of the light rays that fall on it. Diffuse reflector (such as the moon) does not; it becomes a new light source instead, resulting in higher etendue. In short - perfect reflector preserves etendue, but imperfect does not.

The question is to what extend the moon surface is imperfect reflector. For example, if the mirror has a hole, it will be an imperfect reflector, but you can compensate for it using a bigger lense.

By imperfect it's sufficient to be diffuse. You can verify the Moon is diffusive to a good approximation because of the approximately uniform appearance of the full Moon.

The idea is that you can "organize" or "revert" any ray bundle from a system of non-absorbing lenses and specular reflectors, but if your reflector has billions of tiny irregularities it's not viable to build such a system (it's equivalent to an ideal diffuser, in which light is isotropically reflected). The ideal diffusion process is clearly not reversible by itself: if you shine a beam onto a diffuser it spreads the light; if you expose it to the same light (with reversed directions), it again diffuses it instead of reverting to the original beam. In theory again the physical laws of electromagnetism are time reversible, but in practice the effort to revert some systems might be too demanding (you can even do better -- see Maxwell's demon); manipulation of physical apparatus and information acquisition/manipulation itself has a cost that surpasses any gains.

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