Question

A person can't see distinctly at the distance less than one metre . calculate the power of the lens that he should use to read a book at 25cm.

Answer 1

distance of the near point, x' = 100cm
power of the lens, P = ?
focal length of the lens, f = ?
So the focal length, f = x'×d / x'- d
= 100 ×25 / 100 - 25
= 100 × 25 / 75
f = 100 / 3 cm
So the power of a lens, P = 1/f(m)
= 100/100/3
or,. = 3/1
P = 3 D

Answer 2

Answer:

The power of lens is 3 D.

Explanation:

Given,A person can't see distinctly at the less than 1 m = 100 cm

Let distance of the near point be x'= 100 m

Let focal length,f= ?

We know,

$focal \: length \: f = \frac{X' \times d}{X - 'd}$

Here d = Object distance

$f \: = \frac{100 \times 25}{100 - 25}$

$f = \frac{100 \times 25}{75}$

$f = \frac{100}{3} cm$

Let power of lens be P

So,

$P = \frac{1}{f}$

$P = \frac{100}{ \frac{100}{3} }$

$P = 100 \times \frac{3}{100}$

$P = \frac{3}{1}$

$P = 3$

So power of the lens is 3 D