Question

A person can see maximum near distance of $100\text{cm}$. What will be the power of lens needed to read a letter at a distance $20\text{cm}$?

• A. $2\text{D}$
• B. $3\text{D}$
• C. $4\text{D}$
• D. $5\text{D}$

Answer 1

The correct option is C $4\text{D}$

A person can see maximum near distance $100\text{cm}$. It is needed to read at a distance of $20\text{cm}$
i.e.
$u=-20\text{cm}=-0.2\text{m}$ $v=-100\text{cm}=-1\text{m}$
The power of lens required
$P=\frac{1}{f}=\frac{1}{v}-\frac{1}{u}$
$\Rightarrow P=\frac{1}{-1}-\frac{1}{-0.2}$
$\Rightarrow P=-1+5$
$P=4\text{D}$
Hence, option (c) is correct.