Download Human Eye A human eyeball is like a simple camera!

Human Eye
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A human eyeball is like a simple camera!
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Sclera: White part of the eye, outer walls, hard,
like a light-tight box.
Cornea and crystalline lens (eyelens): The two
lens system.
Retina: a thin film of rods and cones located
inside at the back of eyeball.
Anatomy of the Human Eye
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Iris: a colored muscle that acts like
diaphragms to make the pupil larger or
smaller depending on light conditions.
Pupil: a hole in the eye that sllows light to
pass through like a camera aperture.
Eyelid: protects the eye like the lens cover of
a camera.
Sclera (The white/non-transparent tissue surrounding the cornea)
Aqueous humor and Vitreous humor
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The Aqueous Humor is the clear liquid between the
cornea and the lens. It helps regulate the pressure of
the eye ball.
The space that it inhabits is called the anterior
chamber.
The Vitreous Humor is the clear liquid between the
lens and the retina. It is responsible for the shape of
the eyeball.
The space that it fills is called the vitreous body.
Functions?
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Provides nourishment to the eye lens and cornea.
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Hold the shape of the eyeball.
Focusing
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The cornea and eye lens form a compound lens
system, producing a real inverted image on the
retina.
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The eye has a limited depth of field. We cannot see
things close and far at the same time.
Accommodation
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The eye focusing is not done by change the distance
between the lens and retina. Rather, it is done by
changing the focal length of the eyelens!
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Ciliary muscles help to change the shape of the lens.
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Normal eyes can see 25cm to infinity, however, if the cornea bulges
too much or too little. The accommodation does not help. (myopia or
hyperopia)
The Iris
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When it is open as much as it can be, there is
low light level conditions.
When the iris has a small opening, it can cut
down the light intensity by a factor of 20. This
happens during intense light conditions.
Retina Structure
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Light sensitive layer is made of photoreceptors: rods (120 millions) and cones (7
millions) which absorb the light.
Plexiform Layer: nerve cells that process the
signals generated by rods and cones and
relay them to the optical nerve.
Choroid: carries major blood vessels to
nourish the retina and absorb the light so
that it will not be reflected back (dark
pupil!)
Rods and Cones
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Cones: for more precise vision, need strong
light. They help to see colors. Mostly
distributed in the center of the retina
(fovea).
Rods: for peripheral and night vision.
Sensitive to light. Mostly distributed away
from fovea.
Fovea
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The fovea defines the center of the retina, and
is the region of highest visual acuity. The
fovea is directed towards whatever object you
wish to study most closely - this sentence, at
the moment. In the fovea there are almost
exclusively cones, and they are at their
highest density.
Sensitivity
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Cones: slow, fine grain, like color film.
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Need high level of light (photopic condition, day)
High density, high resolution.
Rods: fast, coarse grain, black & white film
Low level of light (scotopic condition, at night)
No color is obvious.
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Adaptation: Changing of retina sensitivity.
http://www.aoa.org/x6024.xml
Vision begins when light rays are reflected off
an object and enter the eyes through the
cornea, the transparent outer covering of the
eye.
The cornea bends or refracts the
rays that pass through a round hole
called the pupil.
The iris, or colored portion of the eye that
surrounds the pupil, opens and closes.
The pupil gets bigger or
smaller to regulate the amount
of light passing through.
The light rays then pass through the lens, which
actually changes shape so it can further bend
the rays and focus them on the retina at the
back of the eye.
The retina is a thin layer of tissue at the back of
the eye that contains millions of tiny light-sensing
nerve cells. The images that we see are projected
onto the retina upside down. Our brain quite
simply, flips the images over so that we see things
upright.
The optic nerve transmits
information to the brain.
The vitreous body gives the eye its
shape.
What are lenses?
Lenses bend light in useful ways. Most
devices that control light have one or more
lenses in them (some use only mirrors, which
can do most of the same things that lenses can
do).
There are TWO basic simple lens types:
Concave and Convex
CONVEX or POSITIVE lenses will
CONVERGE or FOCUS light and
can form an IMAGE.
CONCAVE or
NEGATIVE lenses
will DIVERGE
(spread out) light
rays
Convex lens
The correct name for farsightedness is
Hyperopia. The shape of your eye does not
bend light correctly, resulting in a blurred
image. A convex lens is usually used to
correct this problem.
Concave lens
The correct name of nearsightedness is
myopia. Myopia occurs when the eyeball is
slightly longer than usual from front to back.
This causes light rays to focus at a point in
front of the retina, rather than directly on its
surface. A concave lens is usually used to
correct this problem.