Randall might be correct for conventional optics, but what about metamaterial lenses that break the diffraction limit? https://en.m.wikipedia.org/wiki/Superlens
Can you use a magnifying glass and moonlight to light a fire? (2016)
61–70 of 277 posts
Re: Can you use a magnifying glass and moonlight to light a fire? (2016)
#62Thermodynamics argument seems iffy to me. I can cover the entire surface of the earth with solar panels to harvest the moon light and use the combined electricity to melt iron. If this is ok with thermodynamics then so is using lenses. The rest of the argument seems to be that light cannot be optically condensed to a single point as there will always be some dispersion due to diffraction, and the size and shape of th…
What the thermodynamic principle says you can't do is to take Q units of heat energy from one body and transfer it all to a higher temperature body.
The solar panel system is different - it's a heat engine that takes Q units of heat energy from one body, transfers a portion of it Q₁ to a higher temperature body and transfers the remainder of it (Q - Q₁) to a lower temperature body (the immediate surroundings). That's OK with thermodynamics, subject to upper bounds on the ratio Q₁/Q that depend on the absolute temperatures of the bodies involved.
This suffices to show that the perfect lens system can't work. The dusty lens system can't work because adding dust to a perfect lens doesn't turn it into a heat engine.
Re: Can you use a magnifying glass and moonlight to light a fire? (2016)
#63> 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?
If the light is being radiated from a black body surface, then you cannot make something hotter than that surface. But you're right, the light from the moon is reflected sun light, plenty hot to start a fire. The moon also radiates like a black body, but virtually all that light has longer wavelengths than the reflected visible light. You could start a fire from the light of a single star if you had a big enough lens…
Thunderf00t should totally make a lens to start a fire with starlight.
Re: Can you use a magnifying glass and moonlight to light a fire? (2016)
#64> 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?
[deleted]
Re: Can you use a magnifying glass and moonlight to light a fire? (2016)
#65Thermodynamics argument seems iffy to me. I can cover the entire surface of the earth with solar panels to harvest the moon light and use the combined electricity to melt iron. If this is ok with thermodynamics then so is using lenses. The rest of the argument seems to be that light cannot be optically condensed to a single point as there will always be some dispersion due to diffraction, and the size and shape of th…
I was wrong about what I wrote here and am removing it.
Re: Can you use a magnifying glass and moonlight to light a fire? (2016)
#66Isn't the bigger problem that sunlight is nearly parallel and moonlight is reflected off of a spherical surface?
How are you violating conservation of energy if you're taking all of the light that would hit a square mile of the earth and concentrating it down to the size of a penny?
If you can't concentrate light that way then how do focusing lenses on cutting lasers function? Makes no sense.
Re: Can you use a magnifying glass and moonlight to light a fire? (2016)
#67Isn't the bigger problem that sunlight is nearly parallel and moonlight is reflected off of a spherical surface? How are you violating conservation of energy if you're taking all of the light that would hit a square mile of the earth and concentrating it down to the size of a penny? If you can't concentrate light that way then how do focusing lenses on cutting lasers function? Makes no sense.
It’s not parallel though. The sun and the moon subtend almost exactly the same angle in the sky (which is why solar eclipses work).
Re: Can you use a magnifying glass and moonlight to light a fire? (2016)
#68Thermodynamics argument seems iffy to me. I can cover the entire surface of the earth with solar panels to harvest the moon light and use the combined electricity to melt iron. If this is ok with thermodynamics then so is using lenses. The rest of the argument seems to be that light cannot be optically condensed to a single point as there will always be some dispersion due to diffraction, and the size and shape of th…
What the thermodynamic principle says you can't do is to take Q units of heat energy from one body and transfer it all to a higher temperature body. The solar panel system is different - it's a heat engine that takes Q units of heat energy from one body, transfers a portion of it Q₁ to a higher temperature body and transfers the remainder of it (Q - Q₁) to a lower temperature body (the immediate surroundings). That's…
> you can't do is to take Q units of heat energy from one body and transfer it all to a higher temperature body
Except it's not "heat energy", it's electromagnetic energy that's getting transferred. If the target object is smaller than the source object, you can have an amount of energy that will increase the target's heat above that of the source.
Re: Can you use a magnifying glass and moonlight to light a fire? (2016)
#69> 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.…
The equilibrium temperature of a grey-body is actually independent of its emissivity. A black-blackbody absorbs more radiation than a white-blackbody, but it also emits more. The emissivity factors in both on the absorption and emission side, so the equilibrium temperature is independent of it.
If this seems to fly in the face of all common experience, it's because things that look white aren't actually white in the infrared. Their emissivity is lower in the optical spectrum (where they absorb sunlight) than in the infrared (where they emit), so they are cooler than things that look black to us. In short, they aren't grey-bodies.
The situation is also more complicated because, as you say, that things we are used to also cool by other mechanisms.
Re: Can you use a magnifying glass and moonlight to light a fire? (2016)
#70I'm a bit skeptical of the conclusion and the reasoning process used to arrive there. For example, the article states "In other words, all a lens system can do is make every line of sight end on the surface of a light source, which is equivalent to making the light source surround the target." If you forget about optics for a second, imagine that the outer surface of the sun were wrapped around a point (think of the…
That doesn't work because when the penny is hotter than the sun it radiates an equal amount of energy back to the sun. The sun is a blackbody, it radiates light because it's hot. Once the penny is the same temperature it will radiate back at the sun until the two are in equilibrium.
A penny at a temperature high enough to radiate the same amount of energy as the entire surface of the sun, would be at a far higher temperature than the sun.