Theory:

We will use a set of sign conventions called the New Cartesian Sign Convention when dealing with a light reflection by spherical mirrors. The mirror's pole ($$P$$) is used as the origin in this convention. The x-axis ($$X'X$$) of the coordinate system is taken as the principal axis of the mirror.

The following are the rules to be followed while using sign convention:
• Always place the object to the left of the mirror. This indicates that the object's light strikes the mirror from the left side.
• The mirror's pole is used to measure all distances parallel to the principal axis.
• All distances to the right of the origin (along $$+ x-axis$$) are positive, while those to the left (along $$– x-axis$$) are negative.
• Positive distances are measured perpendicular to and above the principal axis (along $$+ y-axis$$).
• Negative values are assigned to distances measured perpendicular to and below the principal axis (along the $$–y-axis$$).
Sign convention for spherical mirrors

The below table shows the sign convention,