Two convex lenses of focal length `10 cm` and `20 cm` respectively placed coaxially and are separated by some distance d. The whole system behaves like a concave lens. One of the possible value of d is

Options

A. `15 cm`
B. `20 cm`
C. `25 cm`
D. `40 cm`

Answer

B

Explanation

`1/F=1/(f_1)+1/(f_2)-d/(f_1f_2)`
To behave like concave lens, F should be negative.
So, `d/(f_1f_2)gt1/(f_1)+1/(f_2)`
or `d/(f_1f_2)gt(f_1+f_2)/(f_1f_2)`
or `dgt(f_1+f_2) or 30 cm`