and the power of the back surface of the lens is
                 n - 1
       D2 = --------
                    r2
where:
   D2 = back surface power
   n = index of the lens material
   r2 = back surface radius in meters

 and the power of the whole lens is the sum of the front and back surfaces, then the nominal power of the lens should be
                  n - 1          n - 1
       DN = -------- + --------
                    r1             r2
where:
   DN = nominal lens power
   n = index of the lens material
   r1 = front surface radius in meters
   r2 = back surface radius in meters

Well, maybe. Maybe not. We did not talk about the sign of the power of the surface when we were doing surface power. We have two ways of doing this; one way that Opticians do, and a way that Physicists do. We are going to say, "Look at the surface. If it is concave, make the surface power negative. If it is convex, make the surface power positive." That is why, on page (36 / 51), I put +/- in front of each fraction.

If you are following along with this just fine, and want the politically correct version, click on the link. If you are the typical student wanting to learn the basics of this field, DO NOT click on that link.

Using this 'rule', when we are doing just the surface power formula, we will say that the radius of a concave surface is negative, and the radius of a convex surface is positive. Again, the Physics instructors out there just shuddered and wrote off any possibility of ever using this material. That is OK with me!

Read page (36-37 / 51-52), and do the lensmaker's exercises. Check the answers in the back of the book.

Now do a few more. As always, you will only get value from the exercise if you do them before you check. If you are just going to my answers without trying them yourself, the only person you are cheating is yourself.

1. A plastic meniscus lens of index 1.50 has a front radius of curvature of 8 cm and a back radius of curvature of 22.22cm. What is its dioptric power?
2. A barium meniscus lens, n = 1.60, has a front radius of curvature of 12cm and a back radius of curvature of 7.5cm. What is its dioptric power?
3. A flint meniscus lens, n = 1.70, has a front radius of curvature of 11.2 cm and a back radius of curvature of 175 mm. What is its dioptric power?
4. What is the power of a plano-convex polycarbonate lens with a front surface radius of curvature of 500mm?
5. What is the power of a plano-concave crown glass lens with a back surface radius of curvature of 45mm?
6. What is the power of a bi-convex high-lite lens (n = 1.70) with a front surface radius of curvature of 70cm and a back surface radius of curvature of 1.5m?
7. What is the power of a biconcave CR39 lens with a front surface radius of curvature of 250mm and a back surface radius of curvature of 37.5cm?