Concentrating sunlight=hotter magnifier?

It would get as hot as any other piece of glass in the sun, because the energy doesn't get concentrated at all until it's already left the glass.

LOG said:

So, I know that f you can get the angles and distance right with a magnifying lens, you can use it to actually burn things like wood and paper. But I wonder, does the concentrating sunlight also make the glass itself any hotter? Or is the effect only at work once light has passed through the lens?

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If the lens is completely transparent, sunlight won't warm it at all -- it'll be the same temperature as the surrounding air.

But nothing is completely transparent. So the concentrated sunlight will warm the glass a little, but remember two things. One, a lens concentrates sunlight by focusing it, essentially creating a cone of light with its point at the focus. Two, lenses are typically thin. That means that the more concentrated sunlight is always outside and at a distance from the lens. The lens itself is in unconcentrated sunlight. That will warm it a little, but if it's transparent the effect won't amount to much. (However, a lens that is transparent in visual light may be opaque to the thermal infrared wavelengths and they'll warm it.)

Short answer: No, the lens won't be heated by concentrated sunlight.

Glass doesn't get hot in the sun the way, say, a metal surface does, because most of the light is transmitted rather than absorbed. You can test this easily on your car on a sunny day. The metal quite possibly will get so hot you can't put your hand on it without getting burned, but the glass will not. Objects which absorb light re-radiate much of that energy in the infrared range, i.e., as heat.

Glass does not transmit infrared radiation well, which is why a greenhouse, or the interior of your car, gets hot in sunlight. The light wavelength spectrum is transmitted through the glass, heats up whatever non-transmitting surface is inside, and that resultant infrared is trapped, thereby heating everything inside.

blacbird said:

Glass doesn't get hot in the sun the way, say, a metal surface does, because most of the light is transmitted rather than absorbed. You can test this easily on your car on a sunny day. The metal quite possibly will get so hot you can't put your hand on it without getting burned, but the glass will not. ...

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Except for tinted glass. . .