Question

If a concave mirror of focal length 20cm produces a real image at 40cm then what is the distance of the object?

Answer 1

Answer:

$1 \div f = 1 \div u + 1 \div v \\ \\ 1 \div 20 = 1 \div u + 1 \div 40 \\ 1 \div 20 - 1 \div 40 = 1 \div u \\ 2 - 1 \div 40 = 1 \div u \\ 1 \div 40 = 1 \div u \\ \\ u = 40m$

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