The magnifying power of a telescope is `9.` When it is adjusted for parallel rays the distance between the objective and eyepiece is `20cm`. The focal lengths of lenses are

Options

A. `10cm, 10cm`
B. `15cm,5cm`
C. `18cm,2cm`
D. `11cm,9cm`

Answer

C

Explanation

Magnifying power, `m=(f_(0))/(f_(e))=9` …(i)
Where `f_(0)` and `f_(e)` are the focal lengths of the objective and eyepiece respectively
Also, `f_(0)+f_(e)=20cm` …(ii)
On solving (i) and (ii), we get `f_(0)=18cm, f_(e)=2cm`