|
Cornea
The crystal clear dome that covers the front of the eye. The majority
(70%) of the bending (refracting) of light rays is accomplished
by the cornea. The shape of the cornea does not change (with the
exception of small changes that occur offer a lifetime).
Lens
 The
crystalline lens finishes the focusing of light. It helps to "fine
tune" vision, and it is able to change shape to allow focus
on near objects. When in becomes cloudy it is called a cataract.
Unlike the cornea, the lens can be made to change its shape (and
therefore its refractive power) rapidly and voluntarily. Using its
ability to charge shape, the lens allows the eye to change its focal
point. Changes in the shape of the lens will allow a normal eye
to focus on near objects.
Iris
This is the part of the eye that gives it color (i.e. blue, green,
brown). It functions like a shutter in the camera analogy, allowing
more or less light into the eye.
Pupil
This is the opening in the middle of the iris.
Retina
This is a thin layer of nerve tissue that senses light. Specialized
cells called rods and cones convert light energy into nerve signals
that travel through the optic nerve to the brain. The retina is
analogous to the film in a camera.
Fovea
This is the center of the retina that receives the focus of the
object of regard. Nerve cells are more densely packed in this area,
so images that are focused on the fovea can be seen in greater detail.
Optic nerve
This is the nerve that runs from the eyeball to the brain. It carries
information from the retina to the brain for interpretation.
 |
Optics
The term "refractive" refers to the way light rays reflected
from objects in our field of vision are brought into focus within
the eye. The eye's complex optical components intercept, focus,
and process light into nerve impulses, which are sent to the brain—and
that is how we "see." The shape (power) of the cornea,
the power of the crystalline lens inside the eye, and the length
of the eye together determine how rays of life are focused on the
retina, the photosensitive membrane that lines the back wall of
the eye. These three factors must be well matched in order for the
eye to see clearly without correction (emmetropia). If the cornea
is too steeply curved relative to the length of the eye or if the
eye is too long relative to the curvature of the cornea, then myopia
results. If the cornea is too flat or the eye too short, then hyperopia
results. In the picture above, light entering from the left travels
through the cornea, then through the lens and is focused perfectly
on the retina at the fovea. In the normal (emmetropic) eye, distant
objects are focused on the retina. To focus near objects on the
retina, the lens changes its shape—that is, until the lens
becomes stiff with age presbyopia.
|
