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:
Refraction of the light rays
Changing size of the pupils and
Accommodation of the eyes.
A co-ordination of these three processes is necessary for effective
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
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.
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
normal eye (emetropic) light from near and distant objects is focused on the
(Farsightedness) : A near image is focused behind the retina because the eyeball is too short. A biconvex
lens is used to correct this.
(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.