In optical diagrams it is common practice to show incident light rays traveling
from left to right. A convex surface therefore is one in which the center of curvature
C lies to the right of the vertex, while a concave surface is one in which C lies to the
left of the vertex.
If we apply the principle of the reversibility of light rays to the diagrams in
Fig. 3B, we should turn each diagram end-for-end. Diagram (a), for example, would
then become a concave surface with converging properties, while diagram (b) would
become a convex surface with diverging properties. Note that we would then have the
incident rays in the denser medium, i.e., the medium of greater refractive index.