Discuss the sign convention of distances for reflection by a spherical mirror and refraction by a spherical lens.

(N/A) The distances such as object distance $(u)$,image distance $(v)$,focal length $(f)$,and radius of curvature $(R)$ are measured from the pole for a spherical mirror and from the optical centre for a spherical lens.
The sign convention for distances is as follows:
$(1)$ All distances are measured from the pole of the mirror or the optical centre of the lens.
$(2)$ The distances measured in the same direction as the incident light are taken as positive,and those measured in the direction opposite to the direction of incident light are taken as negative.
$(3)$ The heights measured upwards with respect to the $X$-axis and normal to the principal axis ($X$-axis) of the mirror/lens are taken as positive. The heights measured downwards are taken as negative.