One of the lousy things about getting old (prepare yourself!) is that you can be
ID: 306878 • Letter: O
Question
One of the lousy things about getting old (prepare yourself!) is that you can be both near-sighted and farsighted at once. Some original defect in the lens of your eye may cause you to only be able to focus on some objects a limited distance away (near-sighted). At the same time, as you age, the lens of your eye becomes more rigid and less able to change its shape. This will stop you from being able to focus on objects that are too close to your eye (far-sighted). Correcting both of these problems at once can be done by using bi-focals, or by placing two lenses in the same set of frames. An old physicist instructor can only focus on objects that lie at distance between 0.52 meters and 5.3 meters. Assume that the physics instructor would like to have normal visual acuity from 23 cm out to infinity and that his bifocals rest 2.0 cm from his eye.
What is the refractive power of the portion of the lense that will correct the instructors nearsightedness?
Explanation / Answer
Since the person has nearsightedness and he can see only upto 5.3 meters. Therefore, he needs a lens that can let appear the rays from infinity as if coming from a distance of 5.3 meters.
Therefore, the refractive power is
P = -1/5.3 = -0.19D
where D is dioptre.
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