Question

Calculate the focal length of a reading glass of a person if his distance of distinct vision is $75cm$.

• A. 25.6 cm
• B. 37.5 cm
• C. 75.2 cm
• D. 100.4 cm

Answer 1

Answer: (B)

Given,
$V=-75cm$
$u=-25cm$
We know that, lens formula, we have
$\frac{1}{f}=\frac{1}{v}-\frac{1}{u}$
$\frac{1}{f}=\frac{-1}{75}+\frac{1}{25}$
$\frac{1}{f}=\frac{1}{25}-\frac{1}{75}$
$\frac{1}{f}=\frac{3-1}{75}$
$\frac{1}{f}=\frac{2}{75}$
Focal length of a glass,
$f=\frac{75}{2}$
$=375cm$