An astronomical telescope has a magnifying power of `10`. In normal adjustment, distance between the objective and eye piece is `22 cm` calculate focal length of objective lens.

Explanation

Here, m = -10, L = 22 cm
`f_(0)`= ?
As m =`-(I_(0))/(f_(0))`
`-10=-(f_(0))/(f_(0))` or `f_(0)=10f_(e)`
As `L=f_(0)+f_(e)`
`therefore` `22=10f_(e)+f_(e)=11f_(e)`
or `f_(e)=(22)/(11)=2cm`
`f_(0)=10f_(c)=10xx2=20cm`