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A person’s farthest point of clear vision is 30 cm from his eyes. If he wants to read a book held 50 cm away, what power of lens should he use?

1. −1.0 D
2. −1.33 D
3. −1.67 D
4. −2.0 D

Correct answer: −1.33 D

Solution

The lens must image an object at 50 cm to the far point at 30 cm: 1/v - 1/u = 1/f with u = -50 cm, v = -30 cm gives 1/f = -1/30 + 1/50 = -1/75 cm -> f = -75 cm. P = 100/(-75) = -1.33 D.

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