SIGHT

Whenever we are awake, our eyes work constantly to collect information about the world. As this data is analysed by the brain, we are supplied with a detailed picture of our surroundings. We can judge distance, see in dim and bright light, and experience COLOUR VISION. For us to see, light rays reflected by objects around us must meet at the back of the eye. There they trigger electrical signals that are sent to the brain for interpretation.

PARTS OF THE EYE

The eyes sit in two bony cavities in the skull. Light rays entering the eye pass through the cornea, lens, and vitreous humour, before reaching the retina, the light-sensitive area at the back of the eye. Signals generated in the retina leave the eye along the optic nerve and go to the brain. Around each eye lie six tiny muscles, which enable the eye to turn and swivel in its socket.

IMAGE FORMATION

Light rays from an object are refracted (bent) first by the cornea and then by the lens, which can be made to change shape according to the distance of the object from the eye. The refraction of the rays ensure that they meet on the retina. There, images are formed upside down, but the brain makes sense of this information, so we see objects the right way up.

FROM EYES TO BRAIN

Nerve signals leave the eyes in the optic nerves, which meet at the optic chiasma. There, fibres from the inner side of each retina cross so that each side of the brain receives information from each eye. The signals pass along the optic tracts to linked areas at the back of the brain. This part of the brain, called the visual cortex, forms a three-dimensional image of the object being viewed.

CHANGING PUPIL SIZE

CONSTRICTED

In bright light or when viewing close objects, the pupils of our eyes constrict (narrow). This is caused by tightening of circular muscles within the iris, the coloured region of tissue that surrounds the pupil. The constriction of the pupil reduces the number of light rays entering the eye

DILATED

In dim light or when we are viewing distant objects, our pupils dilate (widen). This is due to tightening of a different set of iris muscles that are arranged like spokes in a wheel around the pupil. Full widening of the pupil allows the maximum number of light rays to enter the eye.

COLOUR VISION

The retina houses two types of light-sensitive cells: rods and cones. The cones give us colour vision. There are three different types of cone, each sensitive to light within a different range of light wavelengths (colours). Signals are sent from the cones to the brain. From the overall pattern of signals, the brain can work out the colour of every tiny point in the scene being viewed.

HOW THE RETINA RESPONDS TO LIGHT

When light rays reach the retina, they trigger chemical changes in different light-absorbing substances in the rod and cone cells. These changes trigger electrical signals in the cells. The rods and cones link to a system of connecting nerve cells. These perform some initial processing of the signals and then transmit them along optic nerve fibres to the brain.

RODS AND CONES

In each retina, the rods (seen here coloured grey) outnumber the cones (coloured orange) by about 17 to 1. The cones only work in bright light, whereas rods respond to dim light. Unlike cones, rods are all of the same type. They are responsible for the black-and- white vision we experience in semi-darkness.

Copyright © 2007 Dorling Kindersley

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