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A person may suffer from both myopia and hypermetropia defects.(a) What is this condition called?(b) When does it happen?(c) Name the type of lens often required by the persons suffering from this defect. Draw labelled diagram of such lenses.
(a) This condition is called Presbyopia.
(b) This defect occurs in old age due to the gradual weakening of the ciliary muscle and the eye-lens becoming inflexible (or rigid).
(c) The type of lens often required by the persons suffering from this defect is a bifocal lens.
Explanation
Presbyopia is a defect of vision in which an old person can't see the nearby object clearly (appears blurred), due to the loss of power of accommodation of the eye. The near point of the presbyopic eye gradually recedes and becomes much more than 25 cm away. It is also called a special kind of hypermetropia or old age hypermetropia.
This defect occurs in old age due to the weakening of the ciliary muscle and the eye-lens becoming inflexible (or rigid). Thus, the eye loses its power of accommodation. It is corrected in the same way as hypermetropia by using spectacles containing convex lenses.
It is also possible that a person may suffer from both the defect of vision - myopia as well as hypermetropia. Therefore, these person uses spectacles having bifocal lenses in which upper parts consist of a concave lens (to correct myopia) used for distant vision and the lower part consist of a convex lens (to correct hypermetropia) used for near vision.
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