Question

The distance of distinct vision of a person is 40 cm. Calcuate the power of the lens he should use to read newspaper at a distance of 25 cm.

Answer 1

Answer = 1.5D

Given,

Image distance(v) = -40 cm

object distance (u) = -25 cm

By lens formula,

$\frac{1}{f} = \frac{1}{v} - \frac{1}{u} \\ \frac{1}{f} = \frac{1}{-40} + \frac{1}{25} \\ \frac{1}{f} = \frac{3}{200} \\ f = \frac{200}{3}cm$

Now we know,

Power P =

$\frac{1}{f}(in \: m) \\ = \frac{100}{f} (in \: cm) \\ = \frac{100 \times 3}{200} \\ = 1.5$

Thus the power of the lens is +1.5D

Answer 2

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