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Light enters the eye through the transparent cornea, passing through the pupil at the center of the iris. The lens
adjusts to focus the light on the retina, where it appears upside down and backward. Receptor cells on the retina
send information via the optic nerve to the visual cortex.
Accommodation is not always perfect, and in some cases the light that is hitting the retina is a bit
out of focus. As you can see in Figure 4.8 "Normal, Nearsighted, and Farsighted Eyes", if the
focus is in front of the retina, we say that the person is nearsighted, and when the focus is behind
the retina we say that the person is farsighted. Eyeglasses and contact lenses correct this problem
by adding another lens in front of the eye, and laser eye surgery corrects the problem by
reshaping the eye’s own lens.
Figure 4.8 Normal, Nearsighted, and Farsighted Eyes
For people with normal vision (left), the lens properly focuses incoming light on the retina. For people who are
nearsighted (center), images from far objects focus too far in front of the retina, whereas for people who are
farsighted (right), images from near objects focus too far behind the retina. Eyeglasses solve the problem by adding
a secondary, corrective, lens.
The retina contains layers of neurons specialized to respond to light (see Figure 4.9 "The Retina
With Its Specialized Cells"). As light falls on the retina, it first activates receptor cells known
as rods and cones. The activation of these cells then spreads to the bipolar cells and then to
the ganglion cells, which gather together and converge, like the strands of a rope, forming
the optic nerve. The optic nerve is a collection of millions of ganglion neurons that sends vast
amounts of visual information, via the thalamus, to the brain. Because the retina and the optic