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Question
Due to gradual weakening of ciliary muscles and diminishing flexibility of the eye lens a certain defect of vision arises. Write the name of this defect. Name the type of lens required by such persons to improve the vision. Explain the structure and function of such a lens.
Solution
The defect caused due to gradual weakening of ciliary muscles and diminishing flexibility of the eye lens is presbyopia. Presbyopia is the defect of eye in which a person cannot see nearby objects comfortably and distinctly without corrective eye-glasses. A presbyopic eye has its near point greater than 25 cm and it gradually increases as the eye becomes older.
The type of lens required by such person to improve the vision is bifocal lens.
A bifocal lens consists of both convex lens and concave lenses. The convex lens used in bifocal lens is used to correct hypermetropia (far sightedness) and concave lens is used to correct myopia (short sightedness).
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