Eyes

CONTENTS

Introduction

Light from the noon-time sun looks white. But if a ray of white light is aimed at a prism, a broad band of different colors looking like a rainbow emerges. This color array is called the visible spectrum.

This reflected light passes through the lens and falls on the retina of the eye. Here the light induces nerve impulses that travel through the optic nerve to the brain and then over other nerves to muscles and glands. Our eyes are actually limited to only a very narrow band of light within the electromagnetic spectrum, we see only a fraction of what is really there.

The Electromagnetic Specturm

The spectrum of visible light is only a small part of the much greater spectrum of all electromagnetic radiation (see below). Beyond the blue end lie the invisible ultra violet rays, X-rays and gamma rays . while Intra red microwaves and radio waves lie beyond the redend. All electromagnetic radiation has the power to penetrate matter to a certain extent. High frequency radiation - that is, X-rays and gamma-rays penetrates most.

The eye is similar to a television camera. Both the eye and the television camera convert light energy to electrical energy. The eye converts light to nerve impulses that are interpreted by the brain as the sense perception called sight. A television camera converts light to electronic signals that are broadcast and transformed into light images in a television receiver.
The Visual Field [1] is here divided into four colours. That of each eye [dotted lines] is dissimilar but overlaps [2, 3]. The image from the side is indistinct [4] as it falls on the edges of the retina. The clearest image is formed at the fovea [5]. Information from the retina is taken by the optic nerve fibres [6] to the chasma [7] where "crossing over" takes place. The fibres leave the chasma and form the optic tract [8], which divides into the lateral geniculate body [9]. The visual cortex in the right hemispere [10] receives information from the right side of each eye (the left side of the visual field), and the left hemisphere [11] receives information from the left side of each eye (the right side of the visual field).

The eye is well protected. It lies within a bony socket of the skull. The eyelids guard it in front. They blink an average of once every six seconds. This washes the eye with the salty secretion from the tear, or lachrymal, glands. Each tear gland is about the size and shape of an almond. These glands are situated behind the upper eyelid at the outer corner of the eye. After passing over the eye, the liquid from the gland is drained into the nose through the tear duct at the inner corner of the eye.