The radius of curvature
(R) of a surface is taken as a positive number if it increases the curvature of the waves – i.e.
converges the waves and negative if it decreases the curvature of the ways – i.e. diverges
the waves.
The curvature of the surface, or wave, is defined as 1/radius of that surface or wave. A surface with a small radius of curvature therefore has a large curvature and that with a large radius of curvature a small curvature.