REAL-IS-POSITIVE CONVENTION

REAL-IS-POSITIVE CONVENTION

(1) All distances are measured from the mirror as origin.
(2) Distances of real objects and images are positive.
(3) Distances of virtual objects and images are negative. On the real-is-positive convention a concave mirror has a real principal focus and hence a positive focal length. A convex mirror has a virtual principal focus and a negative focal length.

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