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

Answer

A

Explanation

`(f_(0))/(f_(e))=9, therefore f_(0)=9f_(e)`
Also `f_(0)+f_(e)=20" "(because" final image is at infinity")`
`9f_(e)+f_(e)=20, f_(e )=2cm," "therefore f_(0)=18cm`