471
CHAPTER TWELVE
Nervous System III
Light Refraction
When a person sees an object, either the object is giving off
light, or light waves from another source are refl
ected from
it. These light waves enter the eye, and an image of what is
seen focuses upon the retina. The light rays must bend to be
focused, a phenomenon called
refraction
(re-frak
shun).
Refraction occurs when light waves pass at an oblique
angle from a medium of one optical density into a medium
of a different optical density. For example, as
f gure 12.35
shows, when light passes obliquely from a less-dense
medium such as air into a denser medium such as glass, or
from air into the cornea of the eye, the light is bent toward a
line perpendicular to the surface between these substances.
When the surface between such refracting media is curved,
a lens is formed. A lens with a
convex
surface causes light
waves to converge, and a lens with a
concave
surface causes
light waves to diverge
(f g. 12.36)
. Clinical Application 12.6
discusses some familiar problems with refraction.
The convex surface of the cornea refracts light waves
from objects outside the eye, providing about 75% of the
total refractive power of the eye. The light is refracted again
by the convex surface of the lens and to a lesser extent by
the surfaces of the fl
uids in the eye chambers.
If the shape of the eye is normal, light waves are focused
sharply upon the retina, much as a motion-picture image is
focused on a screen for viewing. Unlike the motion-picture
image, however, the one formed on the retina is upside down
and reversed from left to right
(f g. 12.37)
. When the visual
PRACTICE
37
Explain the origin of aqueous humor and trace its path through
the eye.
38
How is the size of the pupil regulated?
39
Describe the structure of the retina.
Light
wave
Air
Glass
Convex
surface
Concave
surface
Converging
light waves
Diverging
light waves
(a)
(b)
FIGURE 12.36
Light waves passing through a lens. (
a
) A lens with a convex surface causes light waves to converge. (
b
) A lens with a concave
surface causes them to diverge.
Light wave
Perpendicular line
Air
Glass
Refracted
light wave
FIGURE 12.35
When light passes at an oblique angle from air into
glass, the light waves bend toward a line perpendicular to the surface
of the glass.
Light waves
Object
Cornea
Image
Retina
FIGURE 12.37
The image of an object forms upside down on the retina.
previous page 501 David Shier Hole's Human Anatomy and Physiology 2010 read online next page 503 David Shier Hole's Human Anatomy and Physiology 2010 read online Home Toggle text on/off