The magnifying power of a telescope with tube length 60 cm is 5 .

Question:

The magnifying power of a telescope with tube length 60 $\mathrm{cm}$ is 5 . What is the focal length of its eye piece?

1. $20 \mathrm{~cm}$

2. $40 \mathrm{~cm}$

3. $30 \mathrm{~cm}$

4. $10 \mathrm{~cm}$

Correct Option: 4

Solution:

(4) For telescope

Tube length $(\mathrm{L})=f_{o}+f_{e}=60$

and magnification $(m)=\frac{f_{o}}{f_{e}}=5 \Rightarrow f_{0}=5 f_{e}$

$\therefore f_{\mathrm{o}}=50 \mathrm{~cm}$ and $f_{\mathrm{e}}=10 \mathrm{~cm}$

Hence focal length of eye-piece, $f_{\mathrm{e}}=10 \mathrm{~cm}$