
BASIC GEOMETRIC OPTICS 55
is convenient for the lensmaker. In a biconvex lens, the same tool is used for both sides, so it
could be convenient for the signs to be the same. However, we will continue with the conven-
tion that a surface with a positive radius of curvature is convex toward the source. The sign
convention has been the source of many errors in specifying lenses. A diagram is often useful
in communicating with vendors whenever any ambiguity is possible.
We still need to determine the magnication. For the thin lens, a ray from the tip of the
object arrow in Figure 2.17 to the tip of the image arrow will be a straight line. By similar
triangles, ...