The eyes have it

The eye consists of a liquid filled globe, with a transparent window called the cornea at the front and a light-sensitive layer called the retina at the back. In between is a lens made of jelly whose shape can be changed by tiny ciliary muscles muscles inside the eye.

The cornea is curved, and together with the lens, its job is to produce a focused image of whatever we're looking at, on the retina. When we look at objects that are close to us, the ciliary muscles flatten the lens by squeezing it. When we look at distant objects the ciliary muscles relax.

This is why our eyes get tired when we spend too long in front of a computer screen. If you have to spend a long time looking at the screen you should take regular breaks and focus on something further away from time to time.

Myopic, hyperopic, astigmatic, 20/20 vision

This is a vertical section of the right eye shown from the nasal side.

Too much time looking at objects close up means you have a risk of developing myopia, or near-sightedness. This means you cannot see far away objects clearly. Many people squint or narrow their eyes when looking at distant objects. This is an early sign that they may be myopic.

Myopia occurs when the eye focuses the image slightly in front of the retina, because the lens and cornea are too powerful or the eyeball itself is too long. It can be corrected with glasses or contact lenses.

Hyperopia or far-sightedness occurs when people have difficulty seeing objects close up, as when reading small type in a book, and often they can be seen holding things at arms length to read or inspect them. This occurs because the lens and cornea are not powerful enough or the eyeball is too short.

A third problem which affects visual acuity sharpness) is astigmatism. This occurs when the cornea is the wrong shape. It causes blurred vision for both near and far objects but can be corrected by glasses or contact lenses.

A Snellen chart is used to test visual acuity. It was invented by Dr Hermann Snelling, in 1862.

Visual acuity is usually measured using a Snellen chart. It is viewed from a standard distance of six meters (about 20 feet). A person with normal vision is able to read the fourth row from the bottom at this distance. Normal vision is therefore known as 20/20 vision.

A person with better than normal vision may be 20/10 if he can read the second row from the bottom, but a person who can only read down to the sixth row from bottom would have 20/40 vision _ worse than normal.

Other important parts

The colored part of the eye, called the iris, can be made bigger or smaller, changing the size of the pupil, the round opening in the center of the iris through which light passes. Our pupils are bigger or dilated in a darkened room, but are smaller or contracted in bright sunlight.

The retina is the light sensitive part of the eye. The light sensitive cells detect the light that falls on them and produce a small electrical signal that travels to the brain along the optic nerve.

There are two types of light sensitive cells - called rods and cones. There are about 120 million rods in each retina, and although these are the most sensitive they cannot detect different colors. The rods are most useful for seeing at night or for detecting movements at the edge of our vision.

There are also six to seven million cones. These are mostly found in the macula, the part of the retina used when we look directly at an object. At the center of the macula the cones are tightly packed together and it is using this part that we can see the most detail.

There are three types of cone, each of which detects one color of light. There are red cones (64 percent), green cones (32 percent) and blue cones (2 percent). Although there are different numbers of cones we still see these colors equally well, though nobody yet fully understands why. All the other colors that we see are made by combining these three in different amounts. If you look at a TV screen with a magnifying glass you will see that the picture is made up of tiny red, blue and green dots.

Vision and reality

Although the eye is a biologically complex organ, it requires the brain to make sense of the patterns of light it produces on the retina. The part of our brain that deals with vision is called the visual cortex. For example, the image formed on the retina is in fact upside down, but the brain flips the image upside right.

It takes the brain a little bit of time to learn how to use its eyes. This is evident in newborn babies, who start to smile and laugh more as the brain becomes better at focusing the eyes and making sense of the images formed, so that they can recognize faces and return an adult's smile.

Humans also have binocular vision, which means that we can determine how far away an object is. This ability is essential for any physical activity and is something that children learn early through play.

The brain's role in vision can sometimes play tricks on us though. This is what happens when we view an optical illusion. Such illusions were used to great effect by the artist Maurice Esher, as a quick search of Google Images will reveal.

Many people are color blind and cannot tell certain colors apart. The standard test for this is an Ishihara test. You can take one free at www.toledo-bend.com/colorblind/ Ishihara.html .

Everybody has a blind spot in each eye, where the optic nerve joins the retina. Normally we don't notice because the blind spot for each eye is in a different place.

Animal eyes

The octopus has highly developed eyes, but instead of changing the shape of its lens to focus, the octopus' eye moves the lens backwards and forwards.

Insects have compound eyes, which are made up of many individual light sensitive cells on a surface. They cannot see much detail, but they are very good for detecting movement - that's why it's so hard to catch a fly.

Bees and some other insects can see ultraviolet light, something that is invisible to us. Flower petals look very different to them providing directions to the nectar the bee needs.

Birds of prey, such as the golden eagle, have much higher visual acuity than humans. On a Snellen test they would be about 20/0.25, which means they can see things from about 8 times as far away as the best human eyes. However, they do not have the same ability to see in color that we do.

Next time: Making sense of the Chaos theory.

Corrin Funnell is a laser physicist with a specialty in laser spectroscopy. He has taught in the UK, Egypt, at Thailand's own Harrow International School, where he became head of the physics department. Currently, he is head of Physics at Island School, Hong Kong.