Question

A person who can see things most clearly at a distance of $10m$ requires, spectacles to enable to see clearly things at a distance of $30m$. What should be the focal length of the spectacles?

• A. $15cm$
• B. $-15cm$
• C. $10cm$
• D. Zero

Answer 1

The correct option is B $-15cm$

Given u = -30 cm and v = -10 cm

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

$=\frac{1}{-10}-\frac{1}{-30}=-\frac{1}{30}$

$\frac{1}{f}=-\frac{1}{15}$

f = -15 cm

Minus sign signifies that lens used is concave.