Common refractive defects of vision
Sometimes, the eye may gradually lose its power of accommodation. In such conditions, the person cannot see the objects distinctly and comfortably. The vision becomes blurred due to the refractive defects of the eye.
There are mainly three common refractive defects of vision.
These are
(i) myopia or near-sightedness,
(ii) Hypermetropia or far-sightedness,
(iii) Presbyopia.
These defects can be corrected by the use of suitable spherical lenses. We discuss these defects and their correction.
(i) Myopia
Myopia is also known as near-sightedness. A person with myopia can see nearby objects clearly but cannot see distant objects distinctly. A person with this defect has the far point nearer than infinity. Such a person may see clearly upto a distance of a few meters.
In a myopic eye, the image of a distant object is formed in front of the retina [Fig. 11.2 (b)] and not at the retina itself.
This defect may arise due to
(i) excessive curvature of the eye lens, or
(ii) elongation of the eyeball.
This defect can be corrected by using a concave lens of suitable power. This is illustrated in Fig. 11.2 (c).
A concave lens of suitable power will bring the image back onto the retina and thus the defect is corrected.
(ii) Hypermetropia
Hypermetropia is also known as far-sightedness. A person with hypermetropia can see distant objects clearly but cannot see nearby objects distinctly. The near point, for the person, is farther away from the normal near point (25 cm).
N = Near point of a hypermetropic eye.
N’ = Near point of a normal eye.
Such a person has to keep a reading material much beyond 25 cm from the eye for comfortable reading. This is because the light rays from a closeby object are focussed at a point behind the retina as shown in Fig. 11.3 (b).
N = Near point of a hypermetropic eye.
N’ = Near point of a normal eye.
This defect arises either because (i) the focal length of the eye lens is too long, or (ii) the eyeball has become too small. This defect can be corrected by using a convex lens of appropriate power. This is illustrated in Fig. 11.3 (c).
N = Near point of a hypermetropic eye.
N’ = Near point of a normal eye.
eye-glasses with converging lenses provide the additional focusing power required for forming the image on the retina.
(iii) Presbyopia
The power of accommodation of the eye usually decreases with ageing. For most people, the near point gradually recedes away. They find it difficult to see nearby objects comfortably and distinctly without corrective eye-glasses. This defect is called Presbyopia. It arises due to the gradual weakening of the ciliary muscles and diminishing flexibility of the eye lens.
These days, it is possible to correct the refractive defects with contact lenses or through surgical interventions.
Think it over
You talk of wondrous things you see, You say the sun shines bright;
I feel him warm, but how can he Or make it day or night?
– C. CIBBER
Do you know that our eyes can live even after our death? By donating our eyes after we die, we can light the life of a blind person.
About 35 million people in the developing world are blind and most of them can be cured. About 4.5 million people with corneal blindness can be cured through corneal transplantation of donated eyes. Out of these 4.5 million, 60% are children below the age of 12. So, if we have got the gift of vision, why not pass it on to somebody who does not have it? What do we have to keep in mind when eyes have to be donated?
An eye bank collects, evaluates, and distributes the donated eyes. All eyes donated are evaluated using strict medical standards. Those donated eyes found unsuitable for transplantation are used for valuable research and medical education. The identities of both the donor and the recipient remain confidential.
One pair of eyes gives vision to up to FOUR CORNEAL BLIND PEOPLE.