# Do magnifying glasses defy entropy by focusing sunlight?

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Do magnifying glasses defy entropy by focusing sunlight?

Using mirrors and lenses, you could supposedly focus all of the sun's light on an area a few hundred feet across or smaller, defying entropy.

I suppose the target would boil, but maybe it would still work and may still defy entropy.

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Defy entropy ?

What on earth do you mean? Perhaps you could put some numbers or at least equations to it?

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Normally, all sunlight would spread out at the speed of light, but here, It would focus. Perhaps plastic lenses could reduce weight.

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Normally, all sunlight would spread out at the speed of light, but here, It would focus. Perhaps plastic lenses could reduce weight.

But what does that have to do with entropy?

And focused light still travels at the speed of light.

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Same amount of work being accomplished, it is just being accomplished in a smaller area minus whatever losses heating the lens causes.

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Same amount of work being accomplished, it is just being accomplished in a smaller area minus whatever losses heating the lens causes.

I think that's energy, not entropy. Entropy is disorder, right?

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Entropy is the ratio of the quantity of heat transferred across a system boundary to the temperature of transfer.

Edited by studiot

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The answer is, unsurprisingly, no. Lenses do not "defy entropy", which I take to mean that they violate the 2nd law of thermodynamics.

The way that you would see the 2nd law being broken in this case would be by transferring energy from a colder source to a hotter one — that's what is forbidden. And you will never be able to do this. No lens will focus light that makes a spot that's hotter than the source. In this case, you will never be able to get anything hotter than ~6000K, no matter how you configure it, with just passive optics. Somewhere, somehow, you will have to do work if you want to get the target hotter.

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The answer is, unsurprisingly, no. Lenses do not "defy entropy", which I take to mean that they violate the 2nd law of thermodynamics.

The way that you would see the 2nd law being broken in this case would be by transferring energy from a colder source to a hotter one — that's what is forbidden. And you will never be able to do this. No lens will focus light that makes a spot that's hotter than the source. In this case, you will never be able to get anything hotter than ~6000K, no matter how you configure it, with just passive optics. Somewhere, somehow, you will have to do work if you want to get the target hotter.

Here's a good page on the topic. It's detailed... I'll quote a snippet,

Not coincidentally, such a tight focus would also violate the second law of thermodynamics. If you could focus the sun’s rays more tightly than permitted by Liouville’s theorem, it would be possible to create a focal spot hotter than the surface of the sun. This is perfectly consistent with the first law of thermodynamics (conservation of energy), but would immediately violate the second law of thermodynamics, since you could in principle run a heat engine using the focal spot as the "hot" side of the engine, and the surface of the sun as the "cold" (!) side, thereby producing work using only one heat bath (the sun) rather than the conventional two.

To say the same thing in more detail: Let’s take a thermally non-conducting object and place it at the focal point of our system (point E in figure 1). A certain amount of energy falls on the image of the sun. The spot will heat up. Eventually it will become hot enough to glow. The temperature will stabilize at some temperature T such that the re-radiated power just matches the incident power. This temperature cannot be hotter than the surface of the sun; otherwise energy would be flowing from a (relatively!) cooler object to a hotter object, in violation of one of the corollaries of the second law of thermodynamics.

http://www.av8n.com/physics/phase-space-thin-lens.htm

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