Hi, In our class, we were asked whether it's practical to use a magnifying glass to focus sunlight to burn wood. One question really bothers me. Could somebody please explain to me how the focal ratio (focal length of converging lens divided by its diameter) affects the temperature of the wood? It seems to me that only the collecting area (cross-sectional area) of the lens matters. I mean, no matter how long the focal length is, the light rays are still focused to the same point, right? I know how to get the solar flux etc though. My only concern is the focal length. Thanks in advance!