Biomedical Engineering Reference
In-Depth Information
n
′
= 1.52
n
= 1.00
I
O
Figure 3-4.
Light rays diverging from an object (
O
) are converged by a plus spherical
refracting surface to form a real image (
I
).
the amount of divergence or convergence of light rays can be quantified. This is the
basis for the vergence relationship. The vergence of the object rays is added to the
dioptric power of the refractive surface to give us the vergence of the image rays.
This can be expressed as:
object vergence
image vergence
Designating the object vergence as
L
, the surface power as
F
, and the image ver-
gence as
L
+
surface power
=
′
, we have
4
L
+
F
=
L
′
or
L
′
=
L
+
F
The sign of the image vergence tells us whether the image is real or virtual. If the
rays are converging (i.e., positive vergence), the image is real. When the rays
are diverging (negative vergence), the image is virtual.
SAMPLE PROBLEMS
Although the intuitive understanding of optics that we use to help our patients is
often qualitative—not quantitative—in nature, we can't get to this level of under-
standing without solving optical problems. In a certain sense, when it comes to
optics, solving problems is where the rubber meets the road. In this spirit, let's see
how the vergence relationship can be used to solve some basic problems involving
spherical refracting surfaces. In the next chapter, we'll work with lenses.
4. Unfortunately, a plethora of symbols are used to represent object vergence and image vergence.
For example,
U
is sometimes used to represent object vergence and
V
to represent image vergence.
Do not let this confuse you. Just think in terms of object vergence and image vergence rather than
memorizing symbols.
