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Physiology of Vision
In order to achieve clear vision, light reflected from objects is focused onto the retina of both eyes. The processes involved in producing a clear image are:
1. Refraction of the light rays
2. Changing size of the pupils and
3. Accommodation of the eyes.
A co-ordination of these three processes is necessary for effective vision.
1. Refraction of the light rays
When light rays pass from a medium of one density to a medium of a different density they are refracted or bent. This principle is used to focus light on the retina. Before reaching the retina light rays pass successively through the conjunctiva, cornea, aqueous fluid, lens and vitreous body.
The lens is the only structure in the eye that can change its refractive power. Light rays entering the eye need to be bent to focus them on the retina. To focus light rays coming from near objects onto the retina the lens changes itself to a more convex shape with the aid of suspensory ligament and ciliary muscle. The relaxing of the ciliary muscle makes the lens thinner and thereby focuses light rays from distant objects on the retina.
2. Size of the pupils
The size of the pupil controls the amount of light entering the eye. In bright light pupils are constricted. In dim light they are dilated.
The iris consists of one layer of circular and one layer of radiating muscle fibres. Contraction of the circular fibre constricts the pupil. Contraction of the radiating fibres dilates it.
3. Accommodation of the eyes to light
Accommodation is the process whereby light emerging from distant as well as near sources is brought to focus on the retina. In order to focus on near objects within 6 metres, the eye should make the following adjustments.
Constriction of the pupils
Convergence of the eyeballs
Changing the power of the lens
Objects more than 6 metres away from the eyes are focused on the retina without adjustment of the lens or convergence of the eyes.
Functions of the Retina
Light rays falling on the retina causes chemical changes in the photosensitive pigments in the rods and cones. This generates nerve impulses which are conducted to the cerebrum via the optic nerves. The rods are stimulated by dim light and are necessary for night vision.
The cones are sensitive to bright light and colour. Visual purple (rhodopsin) is a photosensitive pigment present only in the rods. It is bleached (degraded) by bright light and is quickly regenerated when an adequate supply of vitamin A is available. This is the visual cycle.
Dark adaptation : When an individual moves into a darkened area where the light intensity is insufficient to stimulate the cones, temporary visual impairment results while the rhodopsin is being regenerated within the rods. When regeneration of rhodopsin occurs, normal sight returns.
Refractive errors of the eye
In the normal eye (emetropic) light from near and distant objects is focused on the retina.
Hypermetropia (Farsightedness) : A near image is focused behind the retina because the eyeball is too short. A biconvex lens is used to correct this.
Myopia (nearsightedness) : The eyeball is too long and distant objects are focused in front of the retina. A biconcave lens is used to correct this.
Astigmatism : This results in blurred vision when there is abnormal curvature of part of the cornea or lens that prevents
focusing on the retina. Cylindrical lenses are used to correct this.
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