Biomedical Engineering Reference
In-Depth Information
10 cm
20 cm
50 cm
Figure 1-8.
Converging light rays have positive vergence. At the distances of 10.00,
20.00, and 50.00 cm, the vergence is
2.00 D, respectively. As the
distance to the point of focus
increases
, convergence
decreases
.
10.00,
5.00, and
+
+
+
What is the vergence of a light source located infinitely far away? The wave-
fronts are flat—they have no curvature—making the vergence equal to zero.
Thinking of it in quantitative terms, the reciprocal of the distance to the object
(infinity) is zero. Or think of it this way: since the light rays are neither diverging
nor converging, the vergence is zero. For clinical purposes, we normally consider
distances greater than 20 ft (or 6 m) as infinitely far away.
REFRACTION AND SNELL'S LAW
The velocity of light depends on the medium in which it is traveling. Light travels
more slowly in an optically dense medium, such as glass, than it does in a less dense
medium, such as air. The degree to which an optical medium slows the velocity
of light is given by its refractive index, which is the ratio of the speed of light in
a vacuum to its speed in the medium. Refractive indices of materials commonly
encountered in clinical practice are given in Table 1-1.
TABLE 1-1.
REFRACTIVE INDICES OF COMMON MATERIALS
Material
Refractive Index
Air
1.000
Water
1.333
Ophthalmic plastic (CR39)
1.498
Crown glass
1.523
Trivex
1.532
Polycarbonate
1.586
Essilor Airwear (plastic)
1.59
Essilor Thin & Lite (plastic)
1.67 or 1.74
