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

Answer

C

Explanation

Here, `M = (f_(0))/(f_(e)) = 9 :. F_(0) = 9 f_(e)` ….(i)
When adjusted for parallel rays, the distance between the objective and eye piece is
`f_(0) + f_(e) = 20 cm`
Using (i), `9 f_(e) = f_(e) = 20 or f_(e) = 2 cm`
From (i), `f_(0) = 9 xx 2 cm = 18 cm`