Question

A person who can see things most clearly at a distance of $10\, cm$ requires spectacles to enable him to see clearly things at a distance of $30\, cm$. The focal length of the spectacles is

• A. 10 cm
• B. 18 cm
• C. 30 cm
• D. 15 cm

Answer 1

Answer: (D)

For viewing far objects, concave lenses are used and for concave lens
$u=$ wants to see $=-30$
$v=$ can see $=-10$
So from $\frac{1}{f} =\frac{1}{v}-\frac{1}{u}$
$\Rightarrow \frac{1}{f} =\frac{1}{-10}+\frac{1}{30}$
$\frac{1}{f} =\frac{-3 +1}{30}$
$f =-15\, cm$