
50 OPTICS FOR ENGINEERS
n, Air
n´, Glass
s´
s
V
S
x
X
S´
x´
X´
Figure 2.13 Magnification with a curved refractive surface. We compute the magnification
using Snell’s law at V.
P =
n
−n
r
. (2.33)
By custom, we use the same letter, P, and the same name, “power,” for both refracting power
and power in the sense of energy per unit time. In most cases, there will be no ambiguity,
because the distinction will be obvious in terms of context and certainly in terms of units.
Refracting power has units of inverse length, and is generally given in diopters = m
−1
.
Continuing our work on Table 2.2, we trace a ray from an object at X, a height x above S,
to V and then to the image, ...