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All distances are measured from the pole of the mirror.
Distances measured in the direction of the incident ray are positive and the distance measured in the direction opposite to that of the incident rays are negative .​
Sign convention:-
To study the reflection of light by spherical mirrors, we follow a set of sign conventions called the New Cartesian Sign Convention. In this convention, the pole (P) of the mirror is taken as the origin. The principal axis of the mirror is taken as the x -axis (XX) of the coordinate system. The conventions are as follows:-
1. The object is always placed to the left of the mirror which implies that the light from the object falls on the mirror from the left-hand side.
2. All distances parallel to the principal axis are measured from the pole of the mirror.
3. All the distances measured to the right of the origin (along +x -axis) are taken as positive while those measured to the left of the origin (along -x -axis) are taken as negative. {Distances measured in the same direction of the incident ray are taken as positive and the distances measured in the direction opposite to the incident ray are taken as negative.}
Distances measured perpendicular to and above the principal axis (along +y -axis) are taken as positive.
4. Distances measured perpendicular to and below the principal axis (along - y -axis) are taken as negative.