You can also watch an animation of the eye at http://pennhealth.com/health_info/
animationplayer/seeing.html.
Figure 20.16: The human eye is a complex structure that detects light; the light passes
through the cornea, pupil, and lens, and is focused on the retina. ( 20 )
Vision involves detecting and focusing light from people and objects. First, light passes
through thecorneaof the eye. The cornea is a clear, protective covering on the outside of
the eye. Next, light passes through the pupil. Thepupilis a black opening in the eye that
lets light enter the eye. Surrounding the pupil is theiris,more commonly brown, blue, grey,
or green.
After passing into the eye through the pupil, light passes next through the lens. Like a hand
lens, thelensof the eye is a clear, curved structure. Along with the cornea, the lens helps
focus light at the back of the eye. This is shown inFigure20.17. The lens must bend light
from nearby objects more than it bends light from distant objects. The lens changes shape
to bend the light by just the right amount to bring objects into focus.
The lens focuses light on theretina,which covers the back of the inside of the eye. The
retina consists of light-sensing cells called rods and cones. Rods let us see in dim light. Cones
let us detect light of different colors. When light strikes rods and cones, it causes chemical
changes. The chemical changes start nerve impulses. The nerve impulses travel to the brain
through the optic nerve (Figure20.16). The brain interprets the nerve impulses and tells
you what you are seeing. You know that the eyes sense light. But do know what light is?
You need to understand the nature of light to fully understand vision.