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The Human Eye

The human eye is a remarkable optical instrument. Optically the eye is like a very fine camera having an elaborate lens system on one side and a sensitive screen, called the retina on the other. The general structure of the eye is shown. It is a horizontal ball of diameter about 2.5 cm.

 The essential parts of the eye, that are relevant for our discussion, are

1. Sclera
It is the outermost coating of the eye ball. It is tough, white and opaque and forms white of the eye. It keeps eye ball in spherical shape and protects it from shocks and injury. It becomes transparent at the front projected part of eye ball called cornea.

2. The Lens System of the Eye
The refracting medium (i.e. the lens system) of the eye consists of the cornea, the aqueous humour, the crystalline lens and the vitreous humour. In front of the eye is the cornea, which is a transparent portion, bulging outwards. Behind the cornea is the crystalline lens, which is held in position by a pair of ciliary muscles. The space between the lens and the retina is filled with a transparent fluid called vitreous humour. The cornea, aqueous humour and vitreous humour are optically denser than air. Their optical density is nearly equal to that of water. But the crystalline lens is optically denser. Thus, when a ray of light enters the eye, it first suffers refraction at the cornea and aqueous humour, then refraction through the lens and the third refraction through vitreous humour. As a result of the three refractions, a real, inverted and diminished image is formed on the sensitive inner surface of the eye, called the retina. Thus, optically, the cornea, aqueous humour, crystalline lens and vitreous humour, together constitute a converging lens system of the eye.

3. The Retina

The sensitive inner surface at the back of the eye is called retina. The real inverted and diminished image formed by the lens system of the eye, is received on the retina. The retina, which plays the role of the photographic film, has two kinds of light detectors called rods and cones, which change light energy into electrical signals. These signals are carried to the brain by nerves. The brain interprets the inverted image as being erect. The most amazing fact that all retinal images are inverted and yet are interpreted by the brain as erect, is not clearly understood yet.

4. The Iris


Behind the cornea is the coloured part of the eye called the iris. It is an opaque diaphragm, which has a central opening, called the pupil. This acts as a stop for the lens. The diameter of the pupil adjusts automatically to control the amount of light entering the eye. The opening of the iris becomes large in dim light and small in bright daylight.

5. Crystalline lens
It is a double convex lens L immediately behind iris. Its back (inner) surface is more convex (6mm) than its front (outer) surface (10 mm). This is made of transparent concentric layers whose optical density increases towards the centre of the lens. The average refractive index of crystalline lens is 1.437. The lens is connected to the sclerotic by the ciliary muscles. The lens thickness is minimum 3.6 mm when muscles are relaxed. The thickness becomes maximum 4 mm, when muscles exert maximum pressure (within the elastic limit).

6. Optic axis
The straight line passing through the centre of the cornea and the lens is called optic axis of the eye.

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