Question

A person cannot see the objects clearly placed at distance more than $40cm$. He is advised to use lens of power:

• A. $-2.5D$
• B. $+2.5D$
• C. $-6.25D$
• D. $+1.5D$

Answer 1

The correct option is A $-2.5D$

That lens should be given to the person which forms the image of an object at infinity at distance 40 cm in front of the eye.

Hence, $u=-\infty$ and $v=-40cm$

Now,

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

$⟹\frac{1}{f}=\frac{-1}{40}-\frac{1}{-\infty }$

$⟹f=-40cm$

$P=\frac{100}{f\left(cm\right)}=\frac{-100}{40}=$

$⟹P=-2.5D$

Answer-(A).