Sign convection for Convex and Concave lens
(i) Convex lenses have a positive focal length.
(ii)The sign convention states that the focal length of a concave lens is negative. The incident light always moves from left to right because the item is always on the left side of the lens.
(iii)All lengths from the right side of the lens to its optical centre are regarded as positive distances.
(iv)Negative distances are those that are measured away from the incident light.
(v) Positive distances are those that are calculated in the same plane as the light’s angles of incidence.
(vi)Height measurements that are positive must be taken perpendicular to the main axis and upward.
(vii)Measured downward and perpendicular to the principal axis, negative heights are those.