How the eye works
What are you looking at? You are staring at a computer screen. Turn and look towards a light and turn back again. Turn and look out of the window and back again. Did you feel anything in your eyes? Probably not, but your eye was making adjustments so that you could keep seeing a clear picture of your surroundings. Let's follow a light ray on its journey from this computer screen into your eye and find out how you can see these words.

The computer screen is a light source. Behind it a beam of tiny particles called electrons is rapidly zigzagging down the screen, making the material in the screen give out light. If we think of a light ray coming from the white space between these letters A B, it travels in a straight line to your eye. The outermost part of your eye is covered by a thin, transparent film called the conjunctiva. The light passes straight through the conjunctiva and reaches the cornea. This is like the window of the eye. It is the transparent form of the eye and bulges outwards. Light rays striking the cornea have their paths changed by it. The light rays are bent inwards towards a black hole in the centre of the eye. When the light ray leaves the cornea it travels through a transparent, watery fluid towards the black hole. Around the black hole is a coloured ring called the iris. The hole itself is called the student. The light ray passes through the student and immediately strikes the lens. This is a transparent structure with sides which bulge outwards. It behaves like the cornea and changes the path of the light rays so that a picture of the view the eye is seeing can be made. The light rays leave the lens and come together or converge as they pass through a transparent jelly on their way to the back of the eye. When our light ray reaches the back of the eye it joins with others to form a picture of the text around A B.

The back of the eye is lined by a layer of material called the retina. This layer is packed with cells which are sensitive to light. The cells in the retina directly behind the lens are cone-shaped cells. They are sensitive to colour. Parts of the retina which are further away from this group of cones, contain rod-shaped cells. These cells are sensitive to the amount of light that enters the eye. The picture which forms on the retina affects both rods and cone cells. It makes them generate tiny amounts of electricity. The rods and cones are connected to tiny nerve fibres which form a network under the retina, which is itself attached to a large nerve at the back of the eye. This nerve is called the optic nerve. The electricity, which is generated by the light sensitive cells, travels along the nerve fibres to the optic nerve then along the optic nerve to the brain. When the brain receives all the electrical signals from the retina it uses them to construct the picture that you see.

Let's return to the exercises you did in the opening paragraph. Although you could not feel it, the eye was making other changes as you looked first towards a light and then out of the window. When you looked towards the light, the eye detected that there was an increase in the number of light rays entering the eye. It responded by making some of the muscles in the iris shortern or contract. This action made the student smaller, so fewer light rays could get in and the view that you could see was still clear.

When you changed from looking at this screen to looking out of the window, something different happened. The lens has a ring of muscle around it. This muscle contracts to increase the way the lens bulges. The extra bulging lens can focus light from nearby objects like your computer screen. When you looked out of the window there were more distant objects in your view. The muscle around the lens relaxed and the fluids in the eye pushed on the lens to make it bulge less. This change allowed the lens to focus light from distant objects so that you could see them clearly.

Finally try one more exercise with your eye. Close your left eye and look at the A. Move your face closer to the screen and you will discover that at some point the B will disappear. The reason for this is the light rays from the B are striking the place where the optic nerve meets the retina. At this place there are no light sensitive cells and the place is called the blind spot. Can you find it?

A B

What is the picture like that forms at the back of the eye?
The picture is called an image. It is upside down and small. Think about it being a bit smaller than a small postage stamp. The part of the image that falls on the blind spot is not detected by any light sensitive cells so the brain receives no information about this part of the image.

How do we see things the right way up?
When the brain gets the information about the image from the signals sent by the cells, it uses this information to turn the image right way up.

Why do we only see one picture when we have two eyes which both make an image?
We do not see two different images although both eyes send information about them to the brain. In the brain the information is used to make one picture. Just sit back and look at this computer monitor. Close one eye, then open it and close the other. You will see that each eye makes an image of the monitor but the images are slightly different owing to the slightly different position of the eyes. The brain blends these two images together to produce one picture, which helps you judge distances. Remove a pen lid , close one eye and then try and put the pen top back on straight away. You may hesitate a little as you do it because you cannot judge distance as accurately with just one eye. Now try the activity again with both eyes open and you will find you can put the pen top on straight away.

What happens to the iris and student when you look away from a bright light into a darker view?
The eye detects that it is receiving fewer light rays and another set of muscles in the iris contracts and makes the student wider so that more light can be let in.

How are the muscles in the iris arranged to make the student change size?
Some muscles are arranged across the radius of the iris. Think of them as arranged like the spokes in a bicycle wheel. When these muscles contract they pull the student wider. The other set of muscles is arranged in circles like rope in a coil. When these muscles contract they make the student smaller. The two sets of muscles do not contract at the same time. When one set of muscles contracts the other relaxes. This also happens in muscles in the arm - the biceps and triceps - which raise and lower the forearm.

What happens to the lens when you turn from a distant view to look at something close by?
The muscle around the lens contracts. This makes the lens bulge which allows light from close by to be focussed on the retina.

Do the muscles that work the iris and the lens also make the eye move in its socket?
No. The eye has three pairs of muscles attached to the outside ? the white part of the eye. They work together, contracting and relaxing to move your eyeball so it can move up and down, from side to side and into other positions in between.

Do other animals have eyes like ours?
A large number of animals do. Fish, amphibians, reptiles, birds and mammals do. The octopus and squid have similar eyes. Insect eyes are made from many small eyes joined together but each eye has a lens. Earthworms have skin, which can detect the presence of light, but they cannot see clearly with it.