Question

The maximum angular magnification produced by an astronomical telescope is $30$. Find the focal length of objective lens, if the focal length of eyepiece is $12.5\text{cm}$.

• A. $25\text{cm}$
• B. $50\text{cm}$
• C. $125\text{cm}$
• D. $250\text{cm}$

Answer 1

The correct option is D $250\text{cm}$

Given:

Maximum angular magnification, $m=30$ and $f_e=12.5\text{cm}$

We know that the maximum angular magnification,

$m=\frac{f_o}{f_e}(1+\frac{f_e}{D})$

$\Rightarrow 30=\frac{f_o}{12.5}(1+\frac{12.5}{25})[\because D=25\text{cm}]$

$\Rightarrow 30=\frac{f_o}{12.5}\times \frac{3}{2}$

$\therefore f_o=250\text{cm}$

Hence, $\left(D\right)$ is the correct answer.