Two convex lenses of focal length `f_1` and `f_2` are mounted coaxially separated by a distance. If the power of the combination is zero, the distance between the lenses is

Options

A. `|f_1-f_2|`
B. `f_1+f_2`
C. `(f_1f_2)/(|f_1-f_2|)`
D. `(f_1f_2)/(f_1-f_2)`

Answer

B

Explanation

`d:` distance between lenses
`(1)/(F)=(1)/(f_1)+(1)/(f_2)-(d)/(f_1f_2)`
`P=(1)/(F)=0`
`(d)/(f_1f_2)=(1)/(f_1)+(1)/(f_2)=(f_2+f_1)/(f_1f_2)`
`d=f_1+f_2`