Cartesian Sign Convention

To keep every mirror formula valid for every possible situation, a single Cartesian sign convention is used throughout: (i) incident light is always taken to travel from left to right; (ii) all distances are measured from the pole of the mirror, which is treated as the origin; (iii) distances measured to the right of the pole, along the principal axis, are taken as positive; (iv) distances measured to the left of the pole are taken as negative; (v) heights measured upward, perpendicular to the principal axis, are taken as positive; (vi) heights measured downward are taken as negative. Applying this one consistent rule set to every quantity -- object distance $u$, image distance $v$, object height $h$, image height $h'$, focal length $f$, radius of curvature $R$ -- is exactly what allows the same mirror equation and magnification formula …