Biomedical Engineering Reference
In-Depth Information
n
l
L
=
1.33
0.33 m
L
=
Since the rays are diverging, we have
L
4.03 D
It is often more convenient to keep the distance in centimeters and compensate
by placing a factor of 100 in the numerator as follows
2
:
(100)(1.33)
33.00 cm
= −
L
=
4.03 D
How do we calculate the vergence of light rays that form an image? Consider the
real image formed by the converging spherical glass surface in Figure 3-2A. The
image, which is formed by light rays that exist in crown glass, is 40.00 cm from
the surface. Image vergence (absolute value) is given by
n
L
= −
′
′
=
L
′
l
where
l
is the index
of the medium in which the rays that form the image are located (i.e., secondary
medium).
Substituting into this relationship, we have
n
′
is the distance from the refracting surface to the image
3
and
n
′
′
′
=
L
′
l
1.52
0.40 m
′
=
L
Since the rays that form the real image are converging, the vergence is designated
with a plus sign as indicated below:
L
3.80 D
It bears repeating that the index is 1.52 because the light rays that form the
image exist in glass. While this may be obvious in this case of a real image, it's not
so obvious, as we'll see, in the case of virtual images.
′
= +
2. If the distance were in millimeters, we could enter a factor of 1000 in the numerator and keep the
distance in millimeters (rather than converting to meters).
3. All sorts of symbols are used to represent object and image distances. Do not let these confuse you.
For example,
u
is sometimes used to represent object distance and
v
to represent image distance.
The concepts are important, not the symbols!
