The lens

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Question

A person can see clearly only up to a distance of 30 cm. He wants to read a book placed at a distance of 50 cm from his eyes. What is the power of the lens he requires for his spectacles?

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Solution

The person is myopic. This defect is corrected by using a concave lens. Putting $u = - 50 cm$ and $v = - 30 cm$ in the lens formula $\frac{1}{v} - \frac{1}{u} = \frac{1}{f}$ gives $f = - 75 cm$ . Therefore, the power of the lens = -1.33 D.

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