Question

Name two major defects of an eye. Explain causes of their development and discuss methods of their correction.

Answer 1

Hint: An eye of a person is said to be impaired (or, defected) when the reflected light rays coming from an object into our eyes do not meet at their desired position. When these light rays meet too soon, then the viewer has nearsightedness and if these rays meet too late, that is beyond the eyeballs, then the viewer has far sightedness.

Complete answer:
As we saw earlier, the two major defects of eyes are nearsightedness and farsightedness.
Myopia is the name of the condition for when a person is suffering from nearsightedness. This type of eye defect arises in a person because the power of the eye becomes too large. This might further be a result of decrease in focal length of the eye lens due to elongation of the eyeball. In this defect, a person is clearly able to see the nearby object but distant objects seem to be blurry because the image is formed before the eye and not on the retina of the eye.
It is therefore corrected using a diverging lens, that is, a concave lens.
Hypermetropia is the name of the condition for when a person is suffering from nearsightedness. This type of eye defect arises in a person because the power of the eye becomes too small. This might further result in increased focal length of the eye lens due to elongation of the eyeball or less curvature of the cornea. In this defect, a person is clearly able to see distant objects but objects close to the eyes appear blurry because the image is beyond the eye and not on the retina of the eye.
It is therefore corrected using a converging lens, that is, a convex lens.
Hence, we saw the two major eyes defects and their correction method.

Note:
Apart from myopia and hypermetropia, there are other eye defects too that are commonly found. These are presbyopia and astigmatism. Presbyopia is the gradual loss of the eye's ability to focus at a point. Astigmatism makes objects look blurry at all distances and is mainly caused by the error in the shape of the cornea.