Two point objects are placed on principal axis of a thin converging lens. One is 20cm from the lens and other is on the other side of lens at a distance of 40 cm from the lens. The images of both objects coincide. The magnitude of focal length of lens is :-

Options

A. `(80)/(3)cm`
B. `(40)/(3)cm`
C. 40cm
D. `(20)/(3)`

Answer

A, C

Explanation

for object `O_(1)O_(1)`
`(1)/(v)+(1)/(20)=(1)/(f)` ......(1)
for object `O_(2)O_(2)`
`(1)/(v)-(1)/(40)=-(1)/(f)` .....(2)

from equation 1 and 2 we get
`f=80//3`