A surface that converges a wavefront is taken a positive e.g. a convex lens A surface that diverges a wavefront is taken as negative e.g. a concave lens The curvature of the surface, or wave, is defined as 1/radius of that surface or wave. A surface with a small radius of ...
Visit BYJU'S for Mirror Equation Formula, sign conventions along with a solved problem. Mirror Equation for convex lens and concave lens along with a detailed diagram and explanation.