What is the mirror formula? How do I use sign conventions correctly?

New Cartesian Sign Convention: All distances are measured FROM the pole (centre) of the mirror. Distances measured in the DIRECTION of incident light (usually left to right) are POSITIVE. Distances measured AGAINST the incident light are NEGATIVE. For a concave mirror: object is placed in front, so u is always negative (object to left of mirror, incident light goes right to left, so measurement in opposite direction is negative). Focal length f is negative for concave mirror (focus is in front). Mirror formula: 1/v + 1/u = 1/f. Example: object at 30cm from concave mirror with f=20cm. u=-30, f=-20. 1/v = 1/f - 1/u = 1/(-20) - 1/(-30) = -1/20 + 1/30 = -1/60. So v = -60cm (image in front, real and inverted).