In my spectacle work, we don't bother with this minus sign-this. It's for the theory-wallahs and their coordinate systems. I just know a concave mirror curves inwards, like a spoon, and it pulls light to a real point in front of it. That's a real focus, solid and useful.
But this sign business? Reminds me of my shop accounts. If I make a profit, it's positive. If I make a loss, like last month when tomatoes rotted, it's negative. So for them, a real focus in front is *positive*. But if the focus is *behind* the mirror—virtual, like in the shaving mirror—that's a loss, a negative. It’s just their way of keeping books.
My life at the lathe tells me the focal length is a real distance, always positive for my work. This negative is only for their pen-and-paper *hasige*.
#exams#study
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