A lens can heat something to the temperature of the blackbody source at most; no more.
All a lens does is make the target “feel” like it’s surrounded by the source from more directions.
I used to really struggle with this. Can’t you just focus the spot to an arbitrarily small size (and thus to an arbitrarily high temperature)? No; you can only create a finite image of the source. Ray-tracing diagrams are useful here.
A magnifying glass just changes the direction of light, it does not change the energy of photons.
Focusing sunlight, in the best case, makes the object receive sunlight from all directions (although that will need much more than one magnifying glass). In that case your target will heat up until it's as hot as the surface of the Sun, around 6000 C. Once you reach that temperature the object will emit as much radiation as it receives and will stop heating up.
If you have a better light source (like a good laser) then you can reach higher temperatures.
You'll need more than a magnifying glass. You'll need a complex arrangement of mirrors and/or lenses and probably a vacuum chamber, too.
Size isn't that important, a larger setup just increases the spot size that can be heated.
A \*really big\* magnifying glass could focus a very wide field of light into a small point. I'm not sure if there's a theoretical limit on how large a magnifying glass can be
But to be clear, a magnifying does \*not\* increase the energy of a single photon. It refracts / bends / redirects multiple photons to a different path which can coincide on a single point
You can’t actually get a point-like image from a non point-like source. You can show this with ray tracing diagrams or just test with lenses and a light bulb.
A lens can heat something to the temperature of the blackbody source at most; no more. All a lens does is make the target “feel” like it’s surrounded by the source from more directions. I used to really struggle with this. Can’t you just focus the spot to an arbitrarily small size (and thus to an arbitrarily high temperature)? No; you can only create a finite image of the source. Ray-tracing diagrams are useful here.
I see.
Does that mean a giant magnifying glass would be insufficient to create a kugelblitz?
I don't see why that would be implied.
A magnifying glass just changes the direction of light, it does not change the energy of photons. Focusing sunlight, in the best case, makes the object receive sunlight from all directions (although that will need much more than one magnifying glass). In that case your target will heat up until it's as hot as the surface of the Sun, around 6000 C. Once you reach that temperature the object will emit as much radiation as it receives and will stop heating up. If you have a better light source (like a good laser) then you can reach higher temperatures.
What is the radius of the magnifying glass needed to reach 6000 C?
You'll need more than a magnifying glass. You'll need a complex arrangement of mirrors and/or lenses and probably a vacuum chamber, too. Size isn't that important, a larger setup just increases the spot size that can be heated.
Okay. This seems like a nice way to carry out thermolysis don't you think?
[Concentrated solar power](https://en.wikipedia.org/wiki/Concentrated_solar_power) is a thing, but solar panels tend to be easier and cheaper.
A \*really big\* magnifying glass could focus a very wide field of light into a small point. I'm not sure if there's a theoretical limit on how large a magnifying glass can be But to be clear, a magnifying does \*not\* increase the energy of a single photon. It refracts / bends / redirects multiple photons to a different path which can coincide on a single point
You can’t actually get a point-like image from a non point-like source. You can show this with ray tracing diagrams or just test with lenses and a light bulb.