
74 OPTICS FOR ENGINEERS
5 diopters. To obtain P
1
+ P
2
= 10 diopters, the second surface has the same power, and the
lens is biconvex. The lens shape is shown along the vertical axis.
3.3.1 Summary of the Thick Lens
• For a thick lens the optical power is approximately the sum of the powers of the
individual surfaces, provided the lens is not extremely thick .
• The spacing of the principal planes of a glass lens is approximately one-third of the
thickness of the lens.
• For a symmetric biconvex or biconcave lens, the principal planes are symmetrically
placed around the center of the lens.
• For a plano-convex or plano-concave lens, one principal plane