Which lens is used for presbyopia? Give reasons.

Defects of vision:

Human eyes have the power of accommodation that enables us to clearly see objects at far as well as near distances. Defects of vision occur does to the loss of power of accommodation, which may be near nearsightedness, far farsightedness, or a combination of both.

1. Nearsightedness is called Myopia, the person with this defect can see nearby objects clearly but is unable to see objects that are far. A concave lens with a focal length equal to the far point of the eye is used to correct this defect.
2. Farsightedness is called hypermetropia. The person with this defect can see far objects clearly but is unable to see nearby objects clearly. The near point of a normal person is 25 cm but for the person with hypermetropia near point increases to a distance greater than 25 cm. It is corrected with the help of a convex lens.
3. Presbyopia: This eye defect develops with age when a person is unable to perceive the clear image of objects at far as well as near distances. Bifocal lenses are used to correct this defect.

Explanation:

1. Presbyopia is a defect of an eye that develops with old age, this is also called old age hypermetropia. In this defect, the eye suffers from both myopia and hypermetropia.
2. To correct this defect a bifocal lens made by a combination of the concave lens(upper part) and convex lens (lower part) is used. A concave lens enables clear seeing objects that are far whereas a convex lens provides a clear vision of objects that are near.