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How do my eyes work?

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How do my eyes work? Your eyes are tough balls of tissue that contain clear jelly. They work by producing an image that is transmitted to your brain as 'sight'. A transparent layer, called the cornea, covers the front of each eye. Below this is the iris, which is a flattened ring of muscle surrounding the pupil.

Light enters the eyeball through the pupil and passes to the lens behind. As the light rays pass through the lens they are bent and focused to form a clear image on the retina. The retina detects this image and turns it into nerve impulses. These impulses travel along the optic nerve to the brain. The brain interprets them as a picture that you can see.

What makes our eyes coloured?

Eye colouring is caused by melanin in the iris.This is the same substance that your skin produces when y ou tan in the Sun. Eyes can be various shades, of brown, blue, grey or green, depending on how much melanin they contain. Brown eyes have much more melanin than blue eyes. You inherit your eye colour from your parents.The colour of the iris is very individual, and it has been suggested that it could be used to identify people in the same way as fingerprints.

The eye works like a sophisticated video camera, tracking moving objects and focusing automatically.The amount of light entering the eye is controlled by the iris.

Why do I blink?

Blinking cleans and lubricates the surface of the eye. The cornea is very sensitive and must be protected from infection and from drying out. Every time you blink, which is normally every few seconds, a film of tears washes the eye's surface clean. It wipes away bacteria and dust, and leaves behind a moist, lubricating layer that also contains substances to kill bacteria.

Blinking is a protective reflex of the body. You can over-rule it for a time and try to stop yourself blinking, but very soon your brain will decide that blinking is essential and you will not be able to stop it from happening. The flow of tears increases when your eyes are irritated, or if a draught causes the surface of the eye to become dry. Excess tears drain away through a duct in the corner of the eye, and pass into the nose.

How do the eyes focus?

The lens bends light to make an image on the retina. The lens can change its shape to make this image sharp. The lens is flexible, and tiny muscles pull on it to alter its shape and focus the image. Your lens is almost round when you are looking at something close up, but it becomes flatter when you look at a distant object.

How do I see in colour?

The retina is packed with a layer of tiny cells called rods and cones. These cells contain coloured substances that react when light falls on them, triggering a nerve impulse. Rods are slim cells that are responsible for black and white vision. They work even in very dim light, seeing everything in shades of grey. Cone cells give us colour vision. They contain different light-sensitive substances that respond to either red, yellow-green or blue-violet light.

Together with the grey images produced from the rods, cone cells give you the coloured picture that you see. Cones only work in bright light, which is why colours are difficult to see in dim light. You have 125 million rod cells and 7 million cone cells in each eye.

Moving pictures

Film and television images consist of a series of rapidly changing still images, yet we see them as continuous motion. There is a slight delay between each of the images that appear on the screen. However, because this delay is so short, our brain is able to fill in the gaps and provide a complete picture of what is happening.

Do my eyes see upside down?

The image that forms on the retina when light passes through the lens is upside down. This is because of the way in which light rays are bent by the eye's lens. The brain automatically turns the image the right way up, but you are never aware that this is happening.

The rods and cones that respond to light entering the eye are buried in the retina, with the nerves connecting them running along the surface. These receptor cells are connected in a regular pattern like electric wiring.

What is colour blindness?

True colour blindness is when people cannot see any colour at all, but it is very rare. More commonly, many people cannot see true colours accurately. The most common form is red-green colour blindness, where people find it difficult to distinguish between red, green and brown. It affects about one in every 12 men, but is less common in women.

Colour blindness is measured using special charts in which patterns are made up with coloured dots. A person who has a colour defect will see these patterns in a totally different way to someone who has perfect colour vision.

Why do some people need glasses?

If the eye is not exactly the right shape, or the lens cannot focus properly, you cannot form a clear image on the retina. In this case you may need to wear glasses to correct your vision. For a short-sighted person, distant objects look blurred because the image forms in front of the retina. A short-sighted person can see nearby objects very clearly.

For a long-sighted person, the image tries to form behind the retina, so it is blurred while the lens tries to focus on a nearby object.

In another sight problem, called astigmatism, the cornea is not evenly curved and vision is distorted. The different sight problems can be overcome with the use of glasses.

People with normal vision will be able to see two shapes in this diagram. Those
who are colour-blind will only be able to see coloured dots.

How do contact lenses work?

Contact lenses are thin plastic discs that rest on the surface of the cornea. They act like the lenses of ordinary glasses. Most modern contact lenses are made from very soft material that does not cause discomfort to the eye. Some lenses are worn for just one day and then thrown away. It can be difficult to get used to wearing contact lenses and to put them in the eye without scratching the delicate cornea. However, many people prefer them to wearing glasses.

Why do some older people need reading glasses?

As people get older, the lenses of their eyes grow harder and cannot change their shapj to focus close up. Reading glasses provide sharp vision close up. Some people wear bifocal glasses, where a small area of the lens is shaped for close-up vision, while the rest of the lens is suitable for distant vision.

Tags: eyes, glasses, brain, nerves

Category: Education  - ( Education Archive)

Date Added: 22 December '11

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