Question

When an object is placed 40cm from a diverging lens, its virtual image is formed 20cm 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 1

Correct Answer - C
We have, `(1)/(f)=(1)/(v)-(1)/(u) rArr (1)/(f)=(1)/(-20)-(-(1)/(40))`
`= (-2+1)/(40)=-(1)/(40)`
or `" " f=-40 cm`
Power of the lens, `P=-(200)/(0.40)=2.5 D`