One formula connects object distance, image distance, and focal length for any mirror — as long as you get the plus and minus signs right.
New Cartesian convention: distances to the left of P are negative. Here both u and v are negative — a real image case.
To solve mirror numericals precisely (not just qualitatively), we use the New Cartesian Sign Convention: the pole is the origin, the object is always placed to the left (so light travels left to right), distances measured to the right of the pole are positive and to the left are negative, and heights measured upward are positive, downward are negative.
With this convention, object distance (u), image distance (v), and focal length (f) are related by the mirror formula: 1/v + 1/u = 1/f. This one equation works for concave and convex mirrors, for every object position — you just have to substitute the correct signs.
Magnification (m) tells you how much bigger or smaller the image is compared to the object: m = h'/h = −v/u. If m is negative, the image is real and inverted; if m is positive, the image is virtual and erect. The size of |m| tells you if it's magnified (>1), same size (=1), or diminished (<1).
Key exam points
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Mirror Formula & Magnification — Class 10 Physics, PYQs + Numericals Explained · CBSE Physics