Question

The objective of an astronomical telescope has a focal length of $4$m and a diameter of $0.25$m. If the magnifying power is $100$, the focal length of the eyepiece is :

• A. $4$cm
• B. $1$cm
• C. $0.06$cm
• D. $25$cm

Answer 1

Answer: (A)

$4$cm

Magnification of a-stronomical telescope es

$m=\frac{f_0}{f_e}$

where $f_0=$ focal length of objective.

$f_e=$ focal length of eyepiece.

Here $m=100$

$\begin{array}{cc}{f}_0& =4m\\ 100=& \frac{4}{f_e}\\ f_e& =0.04m\\ f_e& =4cm\end{array}$

So option $A$ is Correct