Characteristic diagrams showing the refraction of light by convex and concave
spherical surfaces are given in Fig. 3B. Each ray in being refracted obeys Snell's law
as given by Eq. (1m). The principal axis in each diagram is a straight line through the
center of curvature C. The point A where the axis crosses the surface is called the
vertex. In diagram (a) rays are shown diverging from a point source F on the axis
in the first medium and refracted into a beam everywhere parallel to the axis in the
second medium. Diagram (b) shows a beam converging in the first medium toward
the point F and then refracted into a parallel beam in the second medium. F in each
of these two cases is called the primary focal point, and the distance f is called the
primary focallength.
In diagram (c) a