Hedra, Linton asked:
Could a magnifying glass lose its power?
Dave - A magnifying glass works because light goes slower in glass than it does in air. When it hits it, it bends. A magnifying glass is very carefully shaped so all the light which hits it at one point is focussed down into another point. In order to reduce its power as in reduce how closely it can focus would involve changing the shape of the magnifying glass or changing density of air which isn't going to happen very much. You can't change the shape of the glass. What you could do is change how much light is focussed into one place. If you're trying to burn a piece of paper with the magnifying glass – if the glass got scuffed then more of the light would get scattered out and less of it would get focussed down to point. For some definitions of power that could reduce it but not how close it could focus.
Chris - I suppose it's worth bearing in mind that glass is a supercooled fluid. It does flow over time so if you kept your magnifying glass for long enough eventually it might go slightly out of shape and therefore lose its power. That's kind of speculative, isn't it?