When an object is placed (40,cm ) from a diverging lens, its... | Physics | SolveeIt | Solveeit

Today, August 24

Question

When an object is placed $40\,cm$ from a diverging lens, its virtual image is formed $20\,cm$ from the lens. The focal length and power of lens are

A

F=-20 cm, P=-5 D

B

F=-40 cm, P=-5 D

C

F=-40 cm, P=-2.5 D

D

F=-20 cm, P=-2.5 D

Answer

F=-40 cm, P=-2.5 D

Explanation

Solution

$\frac{1}{f}=\frac{1}{v}-\frac{1}{u}$
$\frac{1}{f}=\frac{1}{-20}-\left(-\frac{1}{40}\right)$
$=\frac{-2+1}{40}=-\frac{1}{40}$
$f=-40\, cm$
$P=-\frac{200}{0.40}=-2.5\, D$