Can you use a magnifying glass and moonlight to light a fire? (xkcd What If?)

[pokerfan]

I love this web series (and I even have the book)...

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

—Rogier Spoor

At first, this sounds like a pretty easy question.

A magnifying glass concentrates light on a small spot. As many mischevious kids can tell you, a magnifying glass as small as a square inch in size can collect enough light to start a fire. A little Googling will tell you that the Sun is 400,000 times brighter than the Moon, so all we need is a 400,000-square-inch magnifying glass. Right?



Wrong. Here's the real answer: You can't start a fire with moonlight[1] no matter how big your magnifying glass is. The reason is kind of subtle. It involves a lot of arguments that sound wrong but aren't, and generally takes you down a rabbit hole of optics.

Read the rest: http://what-if.xkcd.com/145/

[thereismore]

1. This is wrong. It's all a question of how well you can focus the amount of light you have.

With the Sun, all you need is a sloppy ole lens, not too big and not too good. With the Moon, with good optics, you can focus the light you have into a really small volume and light it. The Moon, after all, is just a poor mirror to the Sun.

[jeff47]

2. If that is true, I look forward to your perpetual motion machine. (nt)

[thereismore]

3. Non sequitur. nt

[jeff47]

4. Nope. It's the direct result of your claim.

Your claim requires you to create heat, for free.

[thereismore]

6. OK genius.

I have a PhD in physics.

And yes that's how you light something on fire, by concentrating photons in a small volume. The photons reflected off the Moon are much the same as when they left the Sun. So when you collect enough of them and then concentrate them in a small volume, you will create large energy density that will be converted into heat when it is absorbed by something. Make it black velvet. It will catch on fire unless the heat is quickly dissipated somewhere else.

You should know that, for instance, if the Sun is 10,000x brighter than the Moon (don't know the exact number but that's irrelevant), then, because they are pretty much of the same size in the sky, you can get the same energy density in the focus of a 1 cm (across) lens during the day as with a 100 cm lens during the full moon night.

[jeff47]

8. Hrm....if only this was discussed in the article....

Oh wait! It was!! In multiple ways.

[thereismore]

10. OK genius,

100^2 = 10,000

You collect light from the area of the lens. It goes as radius squared.

[Fumesucker]

23. Ever looked at the Moon through a decent telescope, say 150mm dia?

It's brighter than hell at low power, will leave spots in front of your eye for a while but there's no heat to it...

[thereismore]

24. You don't know what heat is. The brightness is a lot of photons. They

carry energy. That energy gets absorbed by the target (if it's black, for example). That energy gets transformed into vibrations of the atomic lattice (phonons). These vibrations are called heat and it will increase the temperature of the target. It does not matter whether the photons came from a hot source or a cold source.

When you look at the Moon through a telescope, you may not "feel" the heat because the photons' energy is converted by retinal rods to neural impulses that are taken into your brain by the optical nerve. But rest assured, in the end their energy is converted to heat that is taken away from the brain by blood.

[Fumesucker]

25. I bet I know more about practical optics than you...

Here's my image of Comet Lovejoy and the Pleiades taken last year..

http://www.democraticunderground.com/103639073

[thereismore]

27. That's a beautiful picture but it isn't a scientific argument. nt

[Mudcat]

5. no smooshing light

As the article describes, roughly, one cannot concentrate light such that the focal point becomes hotter than the source. In this case, the surface of the Moon.

[thereismore]

7. Temperature of the source has little to do with temperature at the focus of the lens.

See my post above.

[jeff47]

9. Then you just violated the second law of thermodynamics.

You just made something hotter than the source, with zero energy input.

[thereismore]

12. Second law of thermodynamics says there is no such process in Nature whose

SOLE result is transfer of HEAT from a colder physical body to a hotter physical body. Look it up, genius.

Of course you can make a small target hotter than the source when the area around the target gets colder (by having photons that normally hit it diffracted and focused on the target instead).

So no, this does NOT violate the 2nd law of thermodynamics because the target getting hotter is not the SOLE result of using the lens. The area around it gets colder. Get it?

[jeff47]

13. And you're ignoring the the source. You don't have infinite light to gather.

And that source is not infinitely hot.

[thereismore]

14. What exactly is the level of your physics education? Seriously. nt

[jeff47]

15. Well, considering your critique of the article is "nuh-uh!!!!!"

I'm not exactly sure of your credentials.

[thereismore]

17. You are becoming incoherent. The second law is about entropy, not temperature. I'll try again:

On a sunny day, you can light something with a lens because you are focusing photons into a small area which leads to an increase of temperature of the target. Now imagine you first reflect the Sun with a mirror, then use the lens. Same thing. What is the temperature of the mirror that just became the source of the light? Colder than the temperature of the surface of the Moon that is reflecting light to us. You see, temperature has nothing to do with it. It's all about how many photons are coming into your target per second (in physics terms, the power of your apparatus). So, with a big enough lens, you can light shit up on a full moon night.

[jeff47]

18. Again, debunked in the article. Try reading it. Then you can argue why the article is wrong. (nt)

[Callmecrazy]

16. Dude, read the article...

Because you're embarrassing yourself. I have a physics degree too and the article explains it in a way I think even you could understand. It'll blow your mind.

[muriel_volestrangler]

29. "photons that normally hit it" - what is that meant to mean?

You make it sound as if there is a 'natural state' of the area around the target, with an entitlement to photons from the moon. No, changing the course of photons from the moon does not make the area around it colder; it just removes a future source of heat for that area.

[thereismore]

30. What it means is that when you stick your lens between the target and the Moon, the lens will focus

the light into the target. As a result, there will be a shadow around the target the size of the lens that will not receive any light from the Moon. Therefore, that area will start to cool and the target will start to get warmer. So as you see, sticking a lens into the light does not just result in heating up the target, there is another effect which is the cooling of everything that is downstream from the lens that is no longer being hit by light.

[muriel_volestrangler]

31. No, the lens does not cool anything

How could it?

Those areas may cool, if they happen to not have other external sources of heat, because they radiate. But those areas, and other sources, are outside this problem. Consider if we were looking at a tiny object in space, and all the light from the Moon, if it didn't hit that object, went on for a light year before encountering matter. You wouldn't say that it would take a year before the effect of inserting the lens between the object and the Moon could have an effect. The effect on the object depends on the Moon and the lens, but not where the Moon's photons might have landed without the lens.

[thereismore]

32. No they are not outside the problem from the point of view of the 2nd Theorem of Thermodynamics.

The key word in it is SOLE. SOLE result. If the sole result of a process is transfer of heat from a colder body to a warmer body, such a process does not exist in nature. It is prohibited. People were arguing you can't warm up a target with a lens because the Moon is cold or some such nonsense.

[muriel_volestrangler]

33. Yes, that is the point - the colder body is the Moon, the warmer body the target

The lens cannot heat up an object to more than the object would be if on the Moon's surface, surrounded by sunlit moonrock.

The process does not transfer heat from "what-the-Moon's-photons-might-have-hit" to the target, either.

[thereismore]

34. No, the argument was you can't transfer heat from the MOON to the target because Moon is cold.

That would be true if that was the SOLE result of such a process. But it isn't. There's also the cooling of the space downstream from the lens.

So heating up a target using photons collected from the Moon is not disallowed by the 2nd Theorem of Thermodynamics.

[muriel_volestrangler]

35. You can heat up a target, but you can't heat it up hotter than the source

We already know the temperature of an object receiving moon photons over an entire half of its surroundings. It's the temperature of the Moon's surface.

Again, and I can't believe that you can't grasp this, the lens does not cool anything at a distance. How could it? There isn't any interaction between the area you're talking about and the lens. The lens doesn't cool an object sitting on Pluto either.

[thereismore]

36. Sorry no time to go on with this. nt

[Mudcat]

22. but it does!

Think of it like this: if you are in a dark room with a candle on one side, and lenses all over the middle, all pointed in a spot on the opposite side of the room, you'll never achieve a focal point hotter than the original candle flame.

[pokerfan]

20. Exactly

That's why there's a limit on the maximum temperature of a solar furnace. Now it's true that the thermodynamic argument rests on the properties of blackbody radiation but the moon is much, much closer to a blackbody than it is to an ideal mirror. Its average albedo is only 12% and it's convex as opposed to concave.

[NeoGreen]

11. Depends on the flashpoint of the fuel...

...given the right conditions, I'd guess...'yes'.

Oh, you are stipulating STP and some woody biomass...then uhhh...IDK, I'd like to conduct some experiments and get back to you.

What's my budget?

[NiteOwl1]

19. Maybe

If you were about 245,000 miles closer to the moon.

[progressoid]

21. I have that book too!

Daughter gave it to me for Xmas.

[dhill926]

26. jesus christ...

this thread is more entertaining than Hillary vs. Bernie.....

[pokerfan]

28. Yeah....

You don't want to get mixed up in physics wars.