The human eye achieves vision by forming an image that stimulates nerve endings, creating the
sensation of sight. Like a camera, the eye consists of an aperture and lens system at the front,
and a light-sensitive surface at the back. Light enters the eye through the aperture- lens system,
and is focused on the back wall.
Horizontal cross section of the human eye.
The lens system consists of two lenses: the corneal lens on the front surface of the eye, and the
crystalline lens inside the eye.
The space between the lenses is filled with a transparent fluid called the aqueous humor. Also
between the lenses is the iris, an opaque, colored membrane.
At the center of the iris is the pupil, a muscle-controlled, variable- diameter hole, or aperture,
which controls the amount of light that enters the eye.
The interior of the eye behind the crystalline lens is filled with a colorless, transparent material
called the vitreous humor.
On the back wall of the eye is the retina, a membrane containing light-sensitive nerve cells
known as rods and cones. Rods are very sensitive to low light levels, but provide us only with
low-resolution, black-and-white vision. Cones allow us to see in color at higher resolution, but
they require higher light levels.
The fovea, a small area near the center of the retina, contains only cones and is responsible for
the most acute vision.
Signals from the rods and cones are carried by nerve fibers to the optic nerve, which leads to
the brain.
The optic nerve connects to the back of the eye; there are no light-sensitive cells at the point
where it attaches, resulting in a blind spot.
Hyperopia
Commonly known as farsightedness, hyperopia is the refractive error in which an image of a
distant object becomes focused behind the retina, either because the eyeball axis is too short,
or because the refractive power of the object is too weak.
This condition makes close objects appear out of focus and may cause headaches, eye strain
and/or fatigue. Squinting, eye rubbing, lack of interest in school and difficulty in reading are
often seen in children with hyperopia.
Eyeglasses or contact lenses may help to correct or improve hyperopia by adjusting the focusing
power to the retina.
Myopia
Commonly known as nearsightedness, myopia is a condition in which an image of a distant
object becomes focused in front of the retina, either because the eyeball axis is too long or
because the refractive power of the object is too strong. This is the opposite of hyperopia.
Myopia is the most common refractive error seen in children.
This condition makes distant objects appear out of focus and may cause headaches and/or
eye strain.
Eyeglasses or contact lenses may help to correct or improve myopia by adjusting the focusing
power to the retina.
Cataract
Cataracts are cloudy areas that form on the lens of your eye. Your lens is a clear, flexible
structure made mostly of proteins (crystallins). As you get older, the proteins in your lens break
down, forming cloudy patches that affect your vision.
Cataract surgery is the only way to remove cataracts and restore your clear vision. During
cataract surgery, an ophthalmologist removes your clouded natural lens and replaces it with an
intraocular lens (IOL).