Biomedical Engineering Reference
In-Depth Information
F
= -10.40 D
n
= 1.52
n
′
= 1.00
+5.00 cm
F
′
C
Figure 2-6.
As light rays travel from more optically dense crown glass to less dense
air, they are refracted away from the surface. Because of its shape, the surface is
diverging. As discussed in the text, the surface's radius of curvature and refractive
index can be used to determine its power. All material to the left of the surface is
assumed to be glass. You can think of this as the back surface of a lens.
where
r
is the surface's radius of curvature in meters,
n
is the index of refraction of
secondary medium, and
n
is the index of refraction of the primary medium.
There are three things you should note about this formula. First, it gives us the
absolute power of the surface. It doesn't tell us if the surface is plus (converging) or
minus (diverging). We should be able to figure that out by tracing rays as we did,
for example, in Figures 2-2 and 2-3. Second, it tells us that radius of curvature and
dioptric power are inversely related—the more curved a surface (or the shorter its
radius), the greater its power. Finally, the greater the difference between the pri-
mary and secondary media, the greater the surface's dioptric power.
′
Let's apply this formula to the spherical surface in Figure 2-6. Light rays are travel-
ing from glass to air, as with the back surface of a lens
.
If the radius of curvature
is 5.00 cm, what is the surface's power?
The first step is to determine if the surface is converging or diverging. Draw-
ing normals to the surface and tracing light rays that are refracted away from the
normal when they enter the less dense medium tells us that the surface diverges
light.
7
Its power is calculated as follows:
n
−
n
′
F
=
r
1.00
1.52
0.05
−
F
=
7. Be sure to compare this surface with that in Figure 2-2, which has the same shape but is converging.
