Question

The near point and far point of a person are $40\text{cm}$ and $200\text{cm}$ respectively. Find the power of the lens, he/she should use while reading a book kept at distance $25\text{cm}$.

• A. $1.5\text{D}$
• B. $-1.5\text{D}$
• C. $3.5\text{D}$
• D. $-3.5\text{D}$

Answer 1

The correct option is A $1.5\text{D}$

If an object is placed at $25\text{cm}$ from the correcting lens. It should produce the virtual image at $40\text{cm}$.

So, $u=-25\text{cm},v=-40\text{cm}$

$\Rightarrow \frac{1}{f}=\frac{1}{v}-\frac{1}{u}$

$\Rightarrow \frac{1}{f}=\frac{-1}{40}+\frac{1}{25}$

$\therefore f=\frac{200}{3}\text{cm}=\frac{2}{3}\text{m}$

$\therefore$ power, $P=\frac{1}{f}=1.5\text{D}$

Hence, $\left(A\right)$ is the correct answer.

Why this question? Concept: To see the nearby objects clearly, image of the object should form at near point and to see far away objects clearly, image should form at far point.