THE EYE

The eye is a complicated structure which works rather like a camera. We see things because light falls on them. Light bounces off objects and passes through a sort of protective windscreen called the conjunctiva, then through another slightly curved transparent membrane called the cornea.

The light rays are bent gently as they pass through the pupil, an adjustable aperture in the centre of the coloured iris, and then go through the lens. The lens shape can change to help us focus on things at different distances. The convex lens turns the light rays  upside –down. When these rays reach the retina, at the back of the eyeball, they are upside- down and back to front. The retina is made up of light-sensitive cells. These stimulate nerve endings, which join together to form the optic nerve. When messages reach the brain, the images are turned the right way up and interpreted.

·        There are two types of cells in the retina, called rods and cones after their shapes. Rods are more sensitive to small amounts of light than cones, but they can only see black and white. Cones pick up colours other than black and white.

Some cone cells see red, some green and some blue. The brain combines their messages to recognize the whole range of visible colours – just as all the colours are made from red, green and blue light on a colour TV screen.