The front surface υ of a meniscus lens is spherical, as shown in Fig. 6.16. The
center of the sphere is at the focus of the lens.
Prove that the expression for the back surface υ of the lens is
r = f (n − 1)n − cos θ (6.48)
where f is the distance between the focus and the front vertex, n the index of refraction of the glass, and θ the angle between the lens axis and a line connecting the
focus and a point S2 on the back surface υ of the lens.