Question

A short-sighted person can see distinctly only those object which lie between $10\text{cm}$ and $100\text{cm}$ from him. The power of the spectacle lens required to see a distant object is

• A. $+0.5\text{D}$
• B. $-1.0\text{D}$
• C. $+1.0\text{D}$
• D. $+4.0\text{D}$

Answer 1

The correct option is B $-1.0\text{D}$

As the person was not able to see object which is away from $100\text{cm}.$ So, we have to use lens of focal length $f$ which can make image of an object which is at infinity to $100\text{cm}$.
$v=-100\text{cm}=-1\text{m}$
using lens maker formula
$\frac{1}{v}-\frac{1}{u}=\frac{1}{f}$
$\Rightarrow \frac{1}{-1}-\frac{1}{\infty }=\frac{1}{f}$
Power of a lens $P=\frac{1}{f}=-1\text{D}$