(Sorry for my bad English, I'm from Germany) Hello! I need to build a solar cooker as a school project. Our build needs to be able to heat 1 liter of water in 80 minutes. My idea was, to heat the water by using a fresnellens. I've bought a 15.6 x 15.6 inches fresnellens for that. I thought about the construction and builded it. Now I have one big question. How am I supposted to heat the water at the focus of the lens? I thought about using a translucent jar with a piece of copper inside of it. The copper is suppost to absorb the sun ray and warm the water inside of the jar. Would this even work? The attempt at a solution Using of copper?