List the 2 causes of presbyopia. draw labelled diagram of a lens used for the correction of this defect of vision.
Answers
Presbyopia= It is the defect of vision due to which an old person cannot see the nearby object clearly due to loss of accomodation of the eye..
Cause of presbyopia=
• Gradual weakening of ciliary muscle.
•Diminishing flexibility of eye lens.
Correction of presbyopia=
• Use of both concave and convex lens of suitable power.
•Such people require bifocal for correction.
•Sometimes a person may suffer from both myopia and hypermetropia..
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Answer:
Presbyopia is special case of hypermetropia.we can call it old age hypermetropia.
In old age , due to weaking of ciliary muscles and eyes lens becomes inflexible [rigid] so the eyes looses its power of accommodation.
Because of this old person cannot see nearby objects clearly.
Presbyopia is corrected using convex lens in spectacles.
it an also happen that a person with this defect cannot see far objects also.
So presbyopia has both defects myopia and hypermetropia.
Correction : A person is advised Bi - focal lens spectacles in which upper part is concave and lower part is convex.