Cataracts and Intraocular Lenses
A cataract is an opacity or "cloudiness" that develops in the lens
of an eye. The result can be serious degradation of vision, or even
blindness. Cataracts can be caused by prolonged exposure to electromagnetic radiation of almost any form. For example, cataracts
are unusually common among airline pilots, who encounter intense UV exposure at high altitude. To delay cataract formation,
doctors recommend protecting the eyes from sunlight with sun-glasses or a hat with a brim.
Cataracts are generally treated by removing the affected lens with
a technique referred to as phacoemulsification. After the natural lens
is removed, it is replaced with a man-made, intraocular lens, or IOL.
In many cases, the IOL is rigid; neither its focal length nor location
can be changed. In most cases these lenses are designed to allow the
eye to see clearly at infinity, but corrective glasses or contacts must be
worn for close vision. In extreme cases, a "piggyback" IOL may need
to be installed along with the original IOL to fine-tune the optics.
More recently, adaptive IOLs have been developed that flex
when the focusing muscles of the eye contract, thus allowing a degree of accommodation. This is illustrated in FlisuRE $27-34,$ where we see the IOL move forward to focus on a close object. Notice that the focal length of the adaptive IOL is fixed, just as with a normal
IOL, but the eye muscles can change its location- the same as in a
camera when it focuses.
After a fixed IOL is installed, it is found to have the wrong
focal length, so a piggyback IOL is installed as well. The piggy-back IOL has a power of -1.50 diopters. What is its focal length
and type?
$$
\begin{array}{ll}{\text { A. converging, } 1.50 \mathrm{m}} & {\text { B. converging, } 66.7 \mathrm{cm}} \\ {\text { C. diverging, } 1.50 \mathrm{m}} & {\text { D. diverging, } 66.7 \mathrm{cm}}\end{array}
$$