This whole piece was a rabbit hole. “Can we start a fire by concentrating light from the moon through a lens?” The answer is no, but the explanation is unintuitive as heck.
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After a chat with light transport fellows, I realized that it is not a well-known fact even among professionals that the entropy in the optical system cannot decrease (2nd law). That’s why you cannot start a fire from a moonlight using just optics what-if.xkcd.com/145/