Introduction to Health Physics: Fourth Edition - Ruang Baca FMIPA UB

Aqueous humor
Cornea
Iris
Lens
Vitreous body
Sclera
Choroid
Macula
Fovea
Retina
BIOLOGICAL BASIS FOR R ADIATION SAFETY 305
Optic nerve
Figure 7-11. The gross structure of the human
eye.
3. The lens—A biconvex, clear, transparent semisolid tissue encased in a transparent
membrane called the capsule; its index of refraction is 1.413. Focusing of
the eye is accomplished through a process called accommodation by thickening
or elongating the lens, thus changing the radii of curvature.
4. The vitreous humor—A soft, jellylike, clear, transparent substance that fills the
eye between the crystalline lens and the retina. It consists of about 99% water
and has an index of refraction of 1.336.
When the eye is illuminated, the light incident on the cornea is concentrated
by the eye’s optical system to form an image on the retina of a much greater light
intensity than that on the cornea. The light enters through the pupil, whose diameter
is varied by the iris diaphragm according to the intensity of the light and the age of
the person. This age dependency is shown in Table 7-3. After passing through the
pupil, the light is focused by the lens on the retina, where the light is transduced
to nerve impulses. These impulses are transmitted to the brain via the optic nerve
(second cranial nerve) to give the sensation of sight.
The retina is a complex structure that is, in fact, an extension of the optic disk, its
point of entry into the interior of the eyeball. On the visual axis, the retina is formed
into a slightly elevated yellow spot, about 0.6 mm in diameter, called the macula lutea;
in the center of the macula is a small, depressed area called the fovea centralis.
TABLE 7-3. Mean Pupil Diameter
AGE DAYLIGHT NIGHT TIME DIFFERENCE
(yrs) (mm) (mm) (mm)
20 4.7 8.0 3.3
30 4.3 7.0 2.7
40 3.9 6.0 2.1
50 3.5 5.0 1.5
60 3.1 4.1 1.0
70 2.7 3.2 0.5
80 2.3 2.5 0.2
Reproduced from Luckiesh M. Moss FK. The Science of Seeing. New York, NY: Van Nostrand; 1937.