According to the Cartesian sign convention, the pole of the mirror is taken as the origin. The principal axis is taken as the X-axis of the frame of reference. The sign conventions are as follows.
- The object is always kept on the left of the mirror. All distances parallel to the principal axis are measured from the pole of the mirror.
- All distances measured towards the right of the pole are taken to be positive, while those measured towards the left are taken to be negative.
- Distance measured vertically upwards from the principal axis are taken to be positive.
- Distance measured vertically downwards from the principal axis are taken to be negative.
- The focal length of a concave mirror is negative while that of a convex mirror is positive.
