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Q: How do lenses that concentrate light not violate the second law of thermodynamics? If you use a magnifying glass to burn ants, aren’t you making a point hotter than the ambient temperature without
The light from one direction (from the direction of the Sun) gets brought together in one position (the focus). So, if the light from the Sun weren’t so parallel we wouldn’t be able to use a lens to focus it so well. A little surprisingly, focusing parallel light to a point does not change the entropy of the light, and there’s a cute way to show that. In this case, were you so inclined, you could put a second lens after the first that “re-parallelizes” the light. There is a general thermodynamic rule which says that you can never focus energy in such a way that the target is hotter than the source.
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