Biomedical Engineering Reference
In-Depth Information
Since the lens is in air, its power is simply the reciprocal of the focal length:
n
f
F
=
1.00
F
=
+
0.20 m
F
= +
5.00 D
SUMMARY
In reality, the thickness of a lens always contributes to its power, but for clinical
applications, it is sometimes acceptable to treat a lens as if its thickness is insignifi-
cant. The power for such a lens, which is called a thin lens, is simply the sum of its
front and back surface powers.
A plus lens forms a real image when the object distance is greater than the focal
length and a virtual image when the object is located closer than the focal length.
Minus lenses always produce a minified virtual image.
The vergence relationship is useful for locating the images formed by thin lenses.
Object and image distances are measured from the plane of the lens. If the lens is
located in air, the primary and secondary media have a refractive index of 1.00.
KEY FORMULAE
Nominal (approximate) power of a thin lens:
F
T
F
2
Lateral magnification produced by a surface or lens:
L
L
=
F
1
+
M
L
=
′
Lateral magnification for a thin lens when there is one medium:
′
l
M
L
=
l
Newton's relation for extrafocal distances
xx
′
=
f
2
SELF-ASSESSMENT PROBLEMS
1. A lens made of crown glass has a front surface with a radius of curvature of
+
8.00 cm and a back surface with a radius of curvature of -6.00 cm. (a) Treating
this lens as a thin lens, what is its power? (b) Calculate the secondary and primary
focal lengths. (c) What is the lens form?
