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A 55-year-old male diabetic has an accommodative power of the lens of 10 dioptres. His near point is located 5 cm (2 in), his far point 10 cm (3.9 in) in front of the eye. Which of the following statements are correct?
(A) his corrective lenses are convex
(B) his corrective lenses have a positive dioptric value
© the patient has hyperopia
(D) the patient is capable of driving a car without corrective glasses
(E) the patient is functionally blind
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Seems like presbyopia
I think a convex lens will do. A
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No crossover.
Its E.
Functionally blind means that a person has a visual impairment, that does not qualify as “legally blind” but results in substantial impediment. With a near point of 5 cm and a far point of 10 cm, the man has a severe case of myopia, not hyperopia as stated in choice C. The total convergence power of the relaxed eye with
normal vision is approximately 60 dioptres, and the cornea accounts for more than two-thirds of that (40 dioptres). The accommodative power of the lens is about 20 dioptres in the very young, about 10 dioptres at age 25, and would be around 1 dioptre at the patient’s age, if he had normal vision. For young adults with normal
vision, the near point is about 10 cm from the eye; the far point is at infinity. The corrective lenses for the myopic eye are concave, not convex (choice A). Concave lenses compensate for the excessive positive dioptres of the myopic eye. These lenses are thin in the middle and wide at the edges and have negative dioptric values, not positive ones (choice B). The patient definitely won’t be able to drive a car or perform other activities that require fast accommodation without corrective glasses (choice D).
