Nervous System III
When a person sees an object, either the object is giving off
light, or light waves from another source are reﬂ
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 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
surface causes light
waves to converge, and a lens with a
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
Explain the origin of aqueous humor and trace its path through
How is the size of the pupil regulated?
Describe the structure of the retina.
Light waves passing through a lens. (
) A lens with a convex surface causes light waves to converge. (
) A lens with a concave
surface causes them to diverge.
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.
The image of an object forms upside down on the retina.