It is my understanding that you can only heat something up to the temerpature the source of heat is. so if i have 10 1 sqft lenses focussing on 1 1sqft lens i would get 10x the energy [roughly] the 1sqft lens would focus. but if i did this in several levels making a million 1sqft of mirrors focussing to a thousand, focussing to 1 I would not get a million times the energy [roughly]
Is this correct?
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-i want to justify a high atmosphere magnifier rendering the Land below into dust. id rather it was a passive effect than needs a solar array and emitters of some kind.
In: Physics
Unfortunately you can’t just stack lenses like that, or rather, it won’t give you the effect you are after.
A magnifying glass can make the Sun look bigger from the point of view of a small spot on the ground. This small spot thus gets more sunlight, and so gets hotter. A large magnifying glass in the sky could do the same for a big area of ground (though still much smaller than the magnifying glass itself). To heat an entire hemisphere would require a magnifying glass many times the diameter of the Earth, presumably floating around in orbit.
[Here](https://what-if.xkcd.com/145/) is a good xkcd explanation.
Technically you could use something called Dyson sphere and get 100% of the sun’s output. You could then redirect this energy any way you see fit. If you want a planet killer, you could do that.
> It is my understanding that you can only heat something up to the temperature the source of heat is.
I don’t believe so. I’m pretty sure the only limiting factors are the size of your reflector (how much energy it can gather) and the efficiency of the mirror.
As a thought experiment, if you set up a dyson sphere of mirrors that collected all the light leaving the sun, and focused all of it down on one car sized asteroid, I can’t see how that would possibly not get hotter than the surface of the sun.
I think the limit you heard probably has to do with just two objects next to each other radiating heat, with no concentration involved, in which case the system would tend towards equilibrium.
Well, firstly lenses aren’t amplifiers. They don’t increase the energy at all, they just focus it. The energy through each lens is 1x as much as it was before. Well, actually less than 1x, because of reflection and absorption. It’s just a higher energy density.
You don’t need multiple lenses. A single lens will do it. With an ideal, coherent light source from a single point and with a perfect lens, you could take whatever light you have, and focus on a single focal point at the right range. This would give you infinite energy density.
Clearly, this does not happen in real life. Adding more lenses won’t make it more possible. Why? A few reasons.
Our lenses aren’t perfect, they have [aberration](https://upload.wikimedia.org/wikipedia/commons/thumb/9/92/Spherical_aberration_2.svg/1010px-Spherical_aberration_2.svg.png), basically meaning they aren’t made perfectly.
Glass causes dispersion, that is seperation of colours, aka a prism making a rainbow. If you perfectly focus red, you won’t be focusing green perfectly. And you can forget focusing infrared, ultraviolet, and microwaves from the sun with a visible light lens.
Most importantly, the sun is not a perfect point light source. You’ll note it isn’t a dot, it has a size meaning light isn’t all coming from the exact same direction. This is why shadows from sunlight are soft rather than sharp, light from the left and right side of the sun aren’t coming from the same angle. What this mean for lenses is that you’ll never be able to focus the entire sun’s light into one point. If you get the right side of the sun onto one spot, the left side will still be somewhere else. The technical term for this is conservation of etendue. If you run the math, the hottest you’ll be able to get something turns out to be the same as the temperature of the sun. About 6000K. If something exceeded 6000K (by other means) and was at the focal point of a very good lens, it’s own thermal radiation output will be higher than the input from the sun through the lens. Meaning, the object would actually start heating the surface of sun backwards through the lens. Insignificantly of course.
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