Question

The distance between an object and the screen is 100cm. A lens produces an image on the screen when the lens is placed at either of the positions 40cm apart. The power of the lens is nearly
A. 3 D
B. 5 D
C. 7 D
D. 9 D

Answer 1

Correct Answer - B
`D=100cm,x=40cm`
`f=(D^2-x^2)/(4D)=((100)^2-(40)^2)/(4xx100)`
`=((100+40)(100-40))/(400)=(140xx60)/(400)=21cm`
`P=(100)/(f(cm))=(100)/(21)=5D`