Question

We combined a convex lens of focal length `f_(1)` and concave lens of focal lengths `f_(2)` and their combined focal length was F . The combination of these lenses will behave like a concave lens, if
A. `f_(1)gtf_(2)`
B. `f_(1)ltf_(2)`
C. `f_(1)=f_(2)`
D. `f_(1)lef_(2)`

Answer 1

Correct Answer - A
(a) For combination of one convex and one concave lens
`1/F=1/(f_(1))-1/(f_(2))rArrF=(f_(1)f_(2))/(f_(2)-f_(1))`
If `f_(1)gtf_(2)`, then F will be negative.