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A farsighted person’s least distance of distinct vision is $0.5 m$ . What is the focal power of the lens that should be prescribed to enable him to read a book held at $0.25 m$ ?

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2.0 D

- 0.4 D

0.4 D

- 2 D

answer is A.

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Detailed Solution

The focal power of the required lens is

+ 2 D

.
The person here is suffering from hyper myopia and as he cannot see nearby objects. The lens he must use is a convex lens. We assume the image distance,

v = - 50 cm

and

u = - 25 cm

Then,

\frac{1}{f} = \frac{1}{v} - \frac{1}{u} \dots (1)

Substituting the values of v and u in the above equation:

\frac{1}{f} = \frac{1}{- 50} - \frac{1}{- 25}

\Rightarrow \frac{1}{f} = \frac{1}{50}

\Rightarrow f = 50 cm

\Rightarrow f = 0.5 m

The power of the convex lens to correct his hyperopia can be calculated as

power, P = \frac{1}{f}

\Rightarrow P = \frac{1}{0.5}

\Rightarrow P = + 2 D

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