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, $f_{\mathrm{e}}=10 \mathrm{~cm}$

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