But before we find the mirror formula, some sign conventions are to be strictly followed. All distances are measured from the pole P. 2. Distances measured in the direction of the incident light rays are taken as positive. Distances measured in the direction opposite to that of the incident rays, is taken as negative. Distance measured upward from the principal axis is taken as positive distance.
Distance measured downward from the principal axis is taken as negative distance.
In a concave mirror or a convex mirror, u is always negative, as PA distance is measured in the direction opposite to that of the light rays striking the mirror surface. In a concave mirror, v can sometimes be positive and sometimes be negative. For convex mirrors, v is always positive. The focus of a concave mirror is considered negative in sign. The focus of the convex mirror, on the other hand is always positive in sign.