I don't want to hijack another thread, so I'm moving the discussion here.... A point about Archimedes Death Ray came up. The question is whether it failed for practical or theoretical reasons. I am of the opinion that it failed for practical reasons due to the inability of a bunch of soldiers to accurately aim a mirror when the spots they project overlap. Particularly at a long distance. Andy, here's a thread where you explain your point: https://www.physicsforums.com/showthread.php?t=224629&highlight=archimedes&page=2 Perhaps the best place to start is I don't understand where this equation comes from: Stepping away from that, the principle, which I agree with from an earlier post, is that you need about 50x the intensity of the sun to get fire. To me, that simply implies 50 mirrors - but to be more realistic (accounting for weather, imperfect mirrors, imperfect focusing) probably a couple hundred. The problem I see with the idea when I draw a diagram is that the sun in a mirror would subtend the same angle as the sun does to the unaded eye, assuming a small distance between the target and mirror compared to sun and mirror. So for best efficiency, you'd want to match the size of the mirror to the angular size of the sun. For the sun at .53 degrees, projected on a target 1 km away, that's 9m per mirror and a 9m spot. Due to the sun being much further from the mirror than the object is, I don't see how the spot can spread out much, so the spot would still be about 9m. Thinking about it another way, a parabolic mirror can be approximated via a bunch of flat plates. The size of the spot projected is a function of the focal length only (as I calculated), so "brightness" would be a function of aperture only.