Question

The magnifying power of a telescope is 9. When it is adjusted for parallel rays, the distance between the eye-piece and objective lens is 20 cm. Find the focal length of both the lenses.

Answer 1

Magnification $\left(m\right)=9$

$m=\frac{f_0}{f_e}\Rightarrow 9=\frac{f_0}{f_c}$

$\Rightarrow f_0=9f_e$

And $f_0+f_e=20$

$10f_e=20\Rightarrow f_e=2cm$

$f_0=2\times 9=18cm$

Hence, the focal length of both lenses are $18cm$ and $2cm$.