The eye

3.6.1 The structure of the eye

Light enters the eye through a thin transparent membrane called the cornea — the most powerful part of the lens system, where the main part of the bending of light occurs. Behind the cornea, the crystalline lens, controlled by the ciliary muscles, adds fine-tuning to the focusing process to produce a sharp image on the retina, which is a photosensitive membrane and acts like an array of miniature photocells. These microscopic units (there are more than 100 million of them) are called rods and cones and respond to light by generating an electrical signal. This signal is carried to the brain by the optic nerve. The light passes through the pupil, an adjustable circular opening in the centre of the iris which regulates the intensity of light entering the eye. The iris is beautifully pig-mented and gives the eye its colour.

The ciliary muscles in the normal human eye are relaxed when one is looking at distant objects, i.e. objects 'at infinity'. The lens then focuses a parallel beam of light on the retina which is about 2 cm from the lens, as illustrated in Figure 3.16.

retina retina

muscles
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