What is presbyopia ? State causes of presbyopia. How is this defect corrected ?

Explanation

Presbyopia is the defect of vision in which aged people find it difficult to see the nearby objects comfortably and distinctly without spectacles.
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Presbyopia is that defect of human eye due to which an old person cannot read and write comfortably.
The power of accommodation of the eye usually decreases with ageing.
For most people, the near point gradually receds away. They find difficult to see nearby objects comfortably and distinctly without corrective eyeglasses.
Sometimes a person may also find it difficult to see distant objects clearly without corrective eyeglasses. In this situation, the far point is shifted from infinity towards the person.
Thus, sometimes, a person may suffer from both myopia and hypermetropia.
Causes of presbyopia : This defect arises due to gradual weakening of the ciliary muscles and diminishing flexibility of the eye lens.
Hence the main cause of presbyopia is only increase in focal length of eye lens whereas the eyeball in presbyopia has normal length.
Correction of the defect: This defect is corrected using a convex lens of suitable power.
A person suffering from both myopia and hypermetropia has to use spectacles of bi-focal lenses, i.e., concave and convex lenses. Normally, the upper portion consists of a concave lens which facilitates distant vision while the lower part is a convex lens which facilitates near vision