Question

A person cannot see objects clearly beyond 2.0 m. The power of lens required to correct his vision will be :

• A. $-\,0.5\text{ }D$
• B. $+\,1.0\text{ }D$
• C. $-\,1.0\,D$
• D. $+\,2.0\,D$

Answer 1

Answer: (A)

$-\,0.5\text{ }D$

Answer 2

When the image of the object which is at infinity, it should be formed at 2 m So, $\frac{1}{v}-\frac{1}{u}=\frac{1}{f}$ or $\frac{1}{f}=\frac{1}{-2}-\frac{1}{-\,\infty }$ $f=-\,2m$ Now the power of lens $P=\frac{1}{f}=\frac{1}{-\,2}=-\,0.5\,D$