By Joe JodoinThe Human Eye

Parts of the eye

There are lots 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 retinaThe 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 diskThe 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 illusionsYour 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

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.

Conduct your own experiment at home!DRAW TWO BLACK SPOTS ON A PIECE OF PAPER

Lines and shapesSome 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 websiteGo to www.michaelbach.de/ot/ this

website contains optical illusions and visual phenomena. You will be surprised how well these optical illusions work.