The least distance of distinct vision of a person is 50 cm. He wants to read a book placed at 25 cm. The focal length of the spectacles would be:
Answer & explanation
Correct answer: option 3
The correct answer is Option (3) → +50 cm
Using len's formula,
$\frac{1}{f}=\frac{1}{v}-\frac{1}{u}$
v = Image distance = -50 cm
u = Object distance = -25 cm
$∴\frac{1}{f}=-\frac{1}{50}+\frac{1}{25}=\frac{1}{50}$
$⇒f=50cm$