Physics - Geometrical Optics Question with Solution | TestHub

Let $f_0$ be the focal length of objective and $f_e$ is the focal length of eyepiece.

Given here, $f_0+f_e=30$.

Angular magnification of telescope is given by $m=\frac{f_0}{f_e}$.

So, $2=\frac{f_0}{f_e}\Rightarrow f_0=2f_e$

Then, we have $f_0+\frac{f_0}{2}=30\Rightarrow \frac{3f_0}{2}=30$

Or, $f_0=20 \mathrm{cm}$.