Recently I've heard that you can't heat up stuff beyond sun's surface temperature using sun+lens combination. It is because you can't transport heat from colder object to a hotter object.
Of course, the bigger lens you use, the closer the heated object gets to the sun's surface temperature. So there's a certain lens size, beyond which you can't get heated object any hotter.
But when I exceed that lens' size… where does the extra heat goes? The lens collects solar power from its entire surface area, and focuses it in one spot. So if you reach certain size you collect lots of solar power, but the heated object doesn't increase the temperature. So where all the extra energy goes?
Best Answer
The size of the lens only determines the intensity of the radiation that shines on the target object, whereas temperature is determined by Planck's law (thermal equilibrium) and hence, also by the amount of radiation emitted from the target object itself. So what you will achieve by shining more intensity on it is that it starts to radiate more itself, so that the additional energy is not "lost", but just bounces back in the direction of the emitter plus in the rest of the half solid angle.