Question

A person has a near point at $80\text{cm}$. Find the power of the lens he should use while reading at $25\text{cm}$.

• A. $2.8\text{D}$
• B. $3.8\text{D}$
• C. $4.8\text{D}$
• D. $5.8\text{D}$

Answer 1

The correct option is A $2.8\text{D}$

If the object is placed at $25\text{cm}$ from the lens, it should produce the virtual image at $80\text{cm}$.

So, $u=-25\text{cm}$ and $v=-80\text{cm}$

Using lens formula,

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

$\Rightarrow \frac{1}{f}=\frac{1}{(-80)}-\frac{1}{(-25)}$

$\Rightarrow \frac{1}{f}=\frac{11}{400}{\text{cm}}^{-1}=\frac{1100}{400}{\text{m}}^{-1}$

$\Rightarrow P=2.8\text{D}$

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