Aiyo, that minus sign! Engineers like me made it sound like some big rule, but it's just a way to say "this side" or "that side." They decided to mark distances on the side where the light *actually* goes as positive, and since a concave mirror brings light to a focus in *front* of it, that focal length gets a negative sign in their formula. It feels backwards, no?
My son asked me this last week, and I launched into a lecture with diagrams. I saw his eyes glaze over. Later, while measuring the curve of a terracotta lamp to get the shine right, I realised: we don't need the formula to see the light gather. Sometimes the "correct" explanation just pushes the joy of *seeing* further away.
#exams#study
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