Structure, Function and Vision Defects
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The cornea is the spherically-shaped
transparent outer layer where light first enters
the eye.
This is the region where refraction of light first
occurs.
The aqueous humor is mostly made up of
water and provides pressure to maintain the
shape of the cornea.
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The iris is the coloured portion of the eye.
The circular opening in the centre of the iris is
called the pupil.
The pupil opens up in order to allow more
light into the interior of the eye when lighting
conditions are poor.
The pupil closes up more in order to block
some of the light in bright conditions.
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The lens is a flexible convex lens made up of
proteins.
The lens changes shape in order to adjust the
focal length for closer or farther objects.
The shape of the lens is controlled by the ciliary
body (which is composed of ciliary muscles
and suspensory ligaments).
The ciliary muscles contract or stretch in order
to change the shape of the lens.
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The ciliary muscles contract to pull the lens
into a rounder shape.
This allows the eye to focus more easily of
nearby objects (accomodation).
As a person ages, their lens becomes harder
and less flexible.
This results in the loss of vision for nearby
objects (the person is now farsighted).
Age related farsightedness (or presbyopia)
generally occurs after age 40.
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The vitreous humor is a liquid/gel-like sac that
contains mostly water.
It provides support for the eyeball and
prevents too much refraction from occurring in
the interior of the eye.
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The retina is the thin layer of cells on the back
of the eye.
It contains light-sensitive cells called Rods and
colour-sensitive cells called Cones.
The image forms on this part of the eye.
The choroid is a dark layer of cells found
behind the retina.
It absorbs excess light, so that it does not
bounce around the interior of the eye.
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The optic nerve attaches to the retina and
carries electrical signals from the eye to the
brain.
Where the optic nerve attaches to the retina
there are no retinal cells (either light or colour
sensitive).
This region is called the blind spot, and each
eye contains one.
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The white part of the eye is called the sclera.
The sclera is the tough outer protective layer of
the eye.
It is very difficult to penetrate this protective
layer.
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The 3 main vision defects are:
Myopia (nearsightedness)
Hyperopia (farsightedness)
Astigmatism
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Occurs when the eye focuses light in front of
the retina.
The person can see objects close by clearly, but
has difficulty seeing objects that are far away.
This occurs when the eyeball is too long.
The correction for this vision defect is a
diverging meniscus lens (which focuses the
light further back, so it reaches the retina).
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Occurs when the eye focuses light behind the
retina.
The person can see far away objects clearly, but
has difficulty seeing objects that are close.
This occurs when the eyeball is too short.
The correction for this vision defect is a
converging meniscus lens (which focuses the
light sooner, so it forms an image on the retina
in front of the back of the eye).
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This vision defect is the result of the cornea
being oval (football shaped) rather than
rounded.
Light from objects refract different amounts
due to the non-symmetrical nature of the
cornea.
Light focuses at different regions in the eye.
A cylindrical lens is used to correct this defect,
which results in blurry images.