Transcript The Human Eye - KaushalGrade10Optics

The Human Eye
By Joe Jodoin
Parts of the eye
There are a lot of parts of the eye,
so EYE will only talk about the main
parts. Those parts are the cornea,
the pupil, the iris, the lens, the
retina, the optic nerve, the optic disk
and the vitreous humor.
The Cornea
The cornea is a clear surface in front
of the eye. It is the first and most
powerful lens in the eye and it contains
no blood vessels. When tears flow over
the cornea and the vitreous humor, it
nourishes the eye. The cornea has five
layers which can be damaged from
accidents, infections, or genetic defects.
The pupil and the iris
The pupil is a round opening below the
cornea and in the middle of the iris that light
passes through. The iris controls its size.
The iris is the colored part of your eye. It
responds to the light around you and can
change the size of the eye. It also helps to
protect the most sensitive part of the retina,
which is called the macula.
The lens
The lens is a transparent part of the eye
that sits behind the iris and the pupil. The lens
can change its shape and size to focus in on
light rays.
The retina
The retina is the part of the eye that
converts light rays in to electronic signals and
sends them to the brain through the optic
nerve. The sides of the retina are responsible
for our peripheral (side) vision, and the center
which is the macula is for our central and
colour vision.
The center of the retina is were most of our
cone cells are, and the sides are were most of
our rod cells are. This is the part of the eye
where most types of vision loss occurs.
The optic nerve and the optic disk
The optic nerve is the cord connecting the
brain to the eye, and it has about 1.2 million
nerve fibers.
The optic disk is the spot on the retina
were the optic nerve leaves the eye. There are
no sensory cells here creating a blind spot but
each eye covers the other eyes blind spot.
The Vitreous humor
The vitreous humor is a jelly-like
substance that fills most of the eye. As
we age it turns from a jelly into a liquid
and gradually shrinks, separating from
the retina.
This is when people start seeing
floaters, dark specs in their vision. This is
a normal sign of aging, but in a few cases
the retina can become detached as the
vitreous separates.
How the eye sees
Vision begins when light reaches our
eyes. The light passes through the
cornea, the pupil and the lens. It
eventually reaches the inner eye where
all visual messages are created. Even
though vision needs light, too much light
can hurt your eye. Luckily, your eyes have
a way to keep this from happening.
The eyes defense mechanism
Your eye has a ring of muscles called the
iris. The iris expands in bright light, and
contracts when in dark places. When the iris
expands, the pupil shrinks, allowing less light
to enter your eyes, so the damage is
reduced. When your iris contracts, the pupil
grows, allowing more light to enter your eye,
so your vision improves.
Rods and Cones
Once light enters the eye, it
travels through the vitreous humor
and then reaches the retina. The
retina is covered with millions of
light sensors called rod cells and
cone cells.
Cone cells
Cone cells are used in bright conditions
and need lots of light to function well. When
bright light enters the pupil and hits the
retina, it “excites” the cone cells and sends
electrical signals to the optic nerve. The
signals travel through the optic nerve to the
brain, where it makes pictures that we can
understand.
Rod cells
In dim conditions cone cells are
almost useless but luckily you have
rod cells to take over. Rod cells work
almost exactly like cone cells but
they get exited in dim conditions like
moonlight and starlight, instead of
bright light.
Colour Vision: Cone cells
Cone cells don’t only sense light. They
also sense colour. There are three main
types of cone cells. One type gets exited
by red, one gets exited by blue, and one
gets exited by green. When the
information of the colours reaches the
brain, it combines the red, green and
blue signals to come up with millions of
different shades.
Colour vision: rod cells
Rod cells work differently than
cone cells. They work slower and get
exited by sensing any light. This
means the rod cells send no colour
info to your brain so that is why you
see only shades of gray in dim
conditions.
Why scientist believe
we need two eyes:
• In case one gets injured
• You get a wider field of vision.(You can see a
little bit sideways as well as straight ahead.)
• They make faint signals more powerful.(Your
brain adds up the signals for stronger images.)
• Your eyes see things from slightly different
angles.(This helps you judge distances and
depths A.K.A Depth Perception)
Optical illusions
Your eyes can sometimes be tricked.
An object might look different than it
actually is. This is called an optical illusion.
A rainbow and TV pictures are both
optical illusions. One is mechanical and
one is natural.
Blind spot
Conduct your own experiment at home!
DRAW TWO BLACK SPOTS ON A PIECE OF PAPER
To find your blind spot close your left eye and
look at the dot on the left and move the page
slowly towards your face. The dot on the right
will disappear when the light reflecting from it
falls on the blind spot in your eye.
Lines and shapes
Some optical illusions make you see curved
lines when they are straight. Some optical
illusions can make things seem bigger, longer
or brighter. Some optical illusions even make it
harder for you to see a steady picture.
Stereogram
A stereogram is a computer generated
image that you can only see if you look at
it in the right way. At first it will just look
like a bunch of dots or squiggles but if
you focus enough you will be able to see
a 3-D image . . .
Optical illusions website
Go to www.michaelbach.de/ot/ this
website contains optical illusions and visual
phenomena. You will be surprised how well
these optical illusions work.