A telephoto combination consists of convex lens of focal length 30cm and a concave lens of focal length 15cm, the separation between two lens is 27.5cm. Where should be the photographic plate placed in order to photograph an object 10m in front of the first lens?

Explanation

For lens `f_(1)`
`f_(1)=30cm, u=10m`
`:. v=30cm` cm
`f_(2)=-10cm, u=2.5cm`
`(1)/(v)-(1)/(25) =(-1)/(15)`
`(1)/(v)=(-1)/(15) -(1)/(2.5)=(1)/(3)rArr v=3cm`