Light Is Focused Using Refraction
and Accommodation
The eye is very similar to a digital camera (Figure
8.9). A digital camera has a lens that can be adjust-
ed to focus the image onto a photosensitive electronic
chip. The chip has an array of picture elements that
transduce the light into electrical impulses that are
combined to form an electronic photograph. In the eye,
the image is focused on the retina, which is packed with
an array of photoreceptors that convert light to action
potentials.
Two processes are involved in focusing an image on the
retina. Refraction or bending of light occurs as the light
passes through areas with different densities. (The various
tissues of the eye have different densities.) Much of the re-
fraction occurs as the light enters the eyeball through the cor-
nea. The amount of refraction is determined by the contour
of the cornea. Most of the remaining refraction occurs at the
lens. Unlike the immobile cornea, the lens can be reshaped
H ow th e ey e form s im ages and d e te c ts light • Figure 8.9
a.
Image formation in the
eye
Light ray before
refraction
Air
Light ray after
Water
I M
I
refraction
Refraction of light rays
1.
Light is refracted as it
enters the eye through
the cornea.
5.
Nerve impulses
are conducted
to the brain via
the optic nerve
(CN I).
4.
The photo-
receptors of
the retina
transduce the
light into nerve
impulses
Nearly parallel rays
from distant object
Lens
Divergent rays
from close object
Lens
3. The light focuses as an
inverted image on the retina.
2.
Ciliary muscles shape the lens
to allow focus on either near or
distant objects (accommodation)
b. Normal and
abnormal
refractions in the eyeball
Plane of focus (anterior to retina)
Concave lens
Nearsighted (myopic) eye,
uncorrected
Nearsighted (myopic) eye,
corrected
Plane of focus (posterior to retina)
Farsighted (hyperopic)
eye, uncorrected
Farsighted (hyperopic)
eye, corrected
NASAL
SIDE
Optic disc
Retinal
blood
vessels
TEMPORAL
SIDE
Macula lutea
Fovea
centralis
View of retina through the pupil of the left eye
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