19.4 Simple Magnifier
If you want to see something better the first thing you might
do is just bring it closer to your eye as illustrated in Figure
19.14. Bringing an object closer to your eye increases the size
of its image on your retina and that allows you to see it in more
detail. But if you bring it too close-closer than your near point-the
lens system of your eye will no longer be able to keep a focused
image on your retina. The image will be fuzzy or blurred.
Figure 19.14 Moving an object closer makes its
image on the retina larger until the object reaches the near point
of the eye. Moving the object still closer makes the image fuzzy
A converging lens can produce an image at infinity that can be
clearly focused by the eye and that will allow a larger image
to be produced on the retina. That is illustrated in Figure 19.15.
There an object is first placed at the near point of the eye-typically
that is about 25 cm away-to produce the largest image possible
with the unaided eye. Then a converging lens is used with the
object positioned at the focal point of the lens. This
produces a virtual image at infinity which can then be viewed
and focused easily by the eye. A sharp and enlarged image is produced
on the retina.
Figure 19.15 A simple magnifier produces a virtual
image at infinity when the object is positioned at the focal point.
It allows the object to be brought closer than the near point
so the clear image formed on the retina will be larger.
For a typical eye with a typical near point of 25 cm, the magnification
offered by a lens used as a simple magnifier is
(need screen capture here)
where f is the focal length of the lens used as a simple magnifier.