Page 325 - Modern Optical Engineering The Design of Optical Systems

304 Chapter Thirteen

(Obviously, very young people can see detail in objects a few inches
from the eye and mature persons whose visual accommodation is fail-
ing may have difficulty focusing on objects several feet away.) The
magnification or magnifying power of a microscope is defined as the
ratio of the visual angle subtended by the image to the angle subtended
by the object at a distance of 10 in from the eye.
The simple microscope or magnifying glass consists of a lens with the
object located at or within its first focal point. In Fig. 13.11, the object
h, a distance s from the magnifier, is imaged at a distance s′ with a
height h′. As shown, the image is virtual and both s and s′ are negative
quantities according to our sign convention. We can readily determine
the magnification by using the first-order equations (2.4 and 2.7) as
follows. The object and image distance equation
1 1 1

s′ f s
is solved for s
fs′
s
f s′

and substituted into the equation for the image height
hs′ h ( f s′)
h′
s f
Now if the eye is located at the lens, the angle subtended by the
image is given by
h′ h (f s′)
′
s′ fs′
If the unaided eye were to view the object at a distance of 10 in,
the angle subtended would be
h

10 in

Figure 13.11 The simple micro-
scope, or magnifier, forms an
erect, virtual image of the object.