DISORDERS OF THE EYE ::
Errors of Refraction ::These errors can be classified as (1) Myopia (2) Hypermetropia and Presbyopia (3) Astigmatism
1.Myopia ::
Myopia persons are shortsighted They can see the near objects perfectly but find difficulty in seeing the distant objects The eyeball is longer than normal and the parallel rays are not focussed on the retina but in the front of the retina The lens may also be more convex This is corrected by wearing bi-concave lenses called negative (-) lens which diverges the parallel rays Those who use their eyes for near objects e.g reading books watch repairing etc use concave glasses Such persons should train their eyes to look at distant objects without glasses
2.Hypermetropia and Presbyopia ::
These persons are longsighted They can see distant objects well but experience difficulty in seeing the near objects The eyeball is shorter than normal Divergent rays from near objects are focussed behind the retina the lens may also remain less convex This rarely occurs in childhood due to disproportionate growth of the eyeball structures and it may get corrected naturally with age
The most common cause of hypermetropia is presbyopia which takes place in old age between 40 and 50 years It is due to gradual loss of elasticity of the lens and weakening of ciliary muscles old lady threading the needle from longer distance is a typical example of presbyopia This is corrected by wearing convex lenses called positive (+) lenses which converage the diverging rays Those who use their eyes for seeing distant objects such as engine drivers use the convex glasses old pepole whose lenses have lost their power of accommodation due to their less elastic lenses suffer from presbyopia and use convex lenses to correct this error of refraction
In another form of presbyopia there is a decreased ability to accommodate for near vision A person suffering from presbyopia holds his book far away in order to see clearly and needs bifocal lens The lower part of these glasses is used for the near vision and the top for the distant vision people who have never worn glasses frequently want reading glasses instead of bifocals as they believe that bifocals will be a difficult adjustment for them However reading glasses will blur their distant vision and they will find that they are constantly taking their reading glasses off and putting them back once again Thus the Short time of adjustment to bifocals is well worth their effort
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