Question

what are sign conventions how they are used

Answer 1

In the case of the spherical mirror, all signs are taken from the pole of the spherical mirror, which is often called origin or origin point. The sign is taken as – (negative) from the pole of a spherical mirror towards object along the principal axis. This means sign is always taken as – (negative) in front of a spherical mirror. For example; the distance of the object is always taken as – (negative) in case of both types of spherical mirror, i.e. concave and convex mirrors. Sign is taken as + (positive) behind the spherical mirror. For example, if an image is formed behind the mirror, the distance of image is taken as + (positive) from the pole along the principal axis. The height of is taken as + (positive) above the principal axis and taken as – (negative) below the principal axis.