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A person can read a book clearly only if he holds it at an arm's length from him. Name the defect of vision:(a) if the person is an old man (b) if the person is a young man
(a) If the person is an old man, then the defect of vision is presbyopia.
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.
(b) If the person is a young man, the defect of vision is hypermetropia.
Explanation
Hypermetropia, also known as long-sightedness or far-sightedness, is a defect of vision in which a person can't see the nearby object clearly (appears blurred), though can see the distant objects clearly. The near point of a hypermetropic eye is more than 25 cm away.
This defect occurs either due to the low converging power of the eye lens, (because of its large focal length). Or, due to the eye-ball being too short, which causes light to focus beyond the retina, instead of directly on the retina.
It is corrected by using spectacles containing convex lenses, which should be of such a focal length (or power) that it forms a virtual image of the object (lying at the normal near point N of 25 cm) at the near point 'N' of the hypermetropic eye.
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