Eye Defects – Myopia,Hypermetropia and Presbyopia

Peter and Paul were sitting on the last bench of a classroom. Peter had problems in seeing the things written and drawn on the board. Everything seemed hazy and unclear to him. So he asked his friend Paul if he saw the same thing, but Paul said that he could see everything clearly. So Peter asked for permission to sit on the front bench. And from there he could see everything written and drawn on the board clearly. So he went to the doctor. He told the doctor that near objects seem prominent but far objects seem hazy and unclear. The doctor told him that he was suffering from a condition known as nearsightedness or myopia. Just like a camera has a lens which focuses and image on the photographic film, similarly in our human eye there is also a lens which focuses the image of an object on the retina. So in normal condition the lens focuses the image on the retina, but let us see what is happening in case of Peter’s eye. See! The image of the blackboard is falling before the retina, not on the retina. The doctor told him that focusing of an image depends on the thickness of the lens. So let us see the difference between the thickness of the lens of Peter and Paul. Notice that, Peter had a problem seeing far objects, like the blackboard. Things written on the blackboard seemed unclear and hazy to him. Now see, his lens is very curved and his eyeball is lenghtened. Now in case of Paul, who had a normal vision when it came to far objects like the blackboard; look at his eye. His lens is relaxed and also his eyeball is off a normal size. Now, there are muscles known as ciliary muscles which help to keep the lens in the proper position. Now all muscles have the property of contraction and relaxation. The ciliary muscles help to stretch the lens.See! and this is done by contraction. By contracting, it facilitates the vision of far off objects by formation of the image of the far off object on the retina. But in case of myopia, what happens is these ciliary muscles get loosened so they are not able to contract properly, and so the lens does not get flattened and hence the image of the far off object does not fall on the retina; but it falls before the retina. And hence, the vision is blurry. So the doctor prescribed a spectacle for Peter. And after wearing specs he could see the board properly. Now the spectacle had concave lens. Now what does a concave lens do? Well, concave lens helps to focus far objects on the retina, not before the retina. And the power of such a glass that contains concave lens is minus. Now, another condition arises in people after 40 years when near objects seem hazy and blurry; like this. This condition when far objects are clear but near objects are hazy and blurry is known as farsightedness or hypermetropia. In this case the lens become flattened and also the eyeball becomes shortened. As a result of which the image does not get focused on the retina, but it gets focused behind the retina. So this can be corrected by convex lens. What does a convex lens do? The convex lens helps in focusing image of a near object on the retina. The spectacles that contain convex lens, has a power plus. Now all the symptoms of hypermetropia are also seen in presbyopia, which occurs in old people. Here, Presbys means means old, and ops means eyes. In presbyopia, like hypermetropia vision of far off objects is prominent but near vision is blurry. Presbyopia is caused by flattening or loosening of the eye lens, and can be corrected by convex lens, like in the case of hypermetropia. But one difference between presbyopia and hypermetropia is that, in hypermetropia the eye defect is caused by both flattening and loosening of eye lens as well as due to shortening of the eyeball.


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