I built a rig with my son using a 65 inch Fresnel lens we harvested from an old rear projection TV. It could easily produce 2,000 degrees F under ideal conditi

I built a rig with my son using a 65 inch Fresnel lens we harvested from an old rear projection TV. It could easily produce 2,000 degrees F under ideal conditions. Focal length was about three fee. A 2x4 passed through the focal point would catch on fire pretty much instantly (as quickly as one could count to one). We successfully fried an egg (more like instantly scorched an egg) by bouncing the beam pre-focal point off a high quality front surface mirror (also harvested from the TV) and focused it onto the bottom of a cast iron skillet. Anyhow, the realization was that this thing was incredibly inconvenient and also dangerous to use. We built it into an articulated wooden frame with the explicit idea of being able to track the sun. Power output peaked and dropped off very quickly and was near useless during low-horizon times. You had to be super aware of where the focal point was at all times or risk a very serious accident. This is an obvious point but you don't fully comprehend how serious a point this is until you start setting wood on fire instantly. Even putting the thing away for the day we had to be sure to rotate it 90 degrees so that the sun hit the edge of the lens. Even then, we threw a black cloth over it just to make sure. This thing could set your house on fire in a fraction of a second. We ended up destroying it after playing with it for a month or so. Just too dangerous to be around and pretty much useless for anything practical. After a while you get tired of demonstrating how quickly you can set a piece of wood on fire. A long intro to say the following. Neat trick by this student and a fantastic learning experience I am sure. Kudo's should be given. Kids learn tons of stuff doing off the wall scientific experiments like this one. In practical terms though, well, it isn't practical at all. But, who cares, this is about learning and on that point it is absolutely brilliant. I remember seeing an article many years ago about using a folded-up t-shirt as a first-pass filter and then simple clear two liter plastic bottles left under the sun for 24 hours to make water safe to drink. People would fill these bottles with t-shirt-filtered water and either place them on a table outside or on the roof for a day. They'd rotate sets of plastic bottles for a continuous supply of safe drinking water. I couldn't think of anything simpler, cheaper or more effective. Here's on version of that: http://www.sodis.ch/methode/anwendung/index_EN http://www.sodis.ch/methode/anwendung/index_EN