What is the focal length of a concave mirror that forms a real, same-size image of an object placed 20 cm from it?
A10 cmCORRECT
B20 cm
C30 cm
D40 cm
GCEPhysics
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Why the answer is A, and why the others tempt you.
Object at centre of curvature (2f) gives a real, same-size image. So 2f = 20 → f = 10 cm.
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