Biomedical Engineering Reference
In-Depth Information
-11.11 D
+10.00 D
-9.00 cm
I
F
O
F
′
-90.10 cm
-1.11 D
Figure 4-10.
The vergence relationship can be used to locate the image formed by
a thin lens. At the top of the lens are the object vergence (
L
) and lens power (
F
T
),
respectively. Below the lens is the image vergence (
L
). Since the object is within the
focal length, the converging lens forms a virtual image.
′
The paraxial relationship gives us
L
′
=
L
+
F
L
′
=
−
11.11 D
+
10.00 D
1.11 D
The negative image vergence tells us that although the light rays' divergence is
diminished subsequent to their refraction by the plus lens, they continue to diverge,
thereby forming a virtual image.
2
Since the lens is surrounded by air (
n
L
′
= −
=
n
′
=
1.00),
the image distance is
n
′
l
′
′ =
L
(100)(1.00)
−
l
′
=
1.11 D
90.10 cm
The virtual image is located 90.10 cm to the left of the thin lens.
As with a spherical surface, the lateral magnification is given by
L
L
l
′
=
−
M
L
=
′
2. As we learned in Chapter 3, an image formed by rays with negative vergence (i.e., diverging rays)
is always virtual, and an image formed by rays with positive vergence (converging rays) is always real.
