Question

The angular magnification of astronomical telescope of distant object is 5. If the distance between objective and eye piece is 36 cm and the final image is formed at infinity then the focal length of its objective and eye piece will be

• A. $f_o=30cm$ and $f_e=6cm$
• B. $f_o=.72cm$ and $f_e=3cm$
• C. $f_o=30cm$ and $f_e=10cm$
• D. $f_o=50cm$ and $f_e=10cm$

Answer 1

The correct option is A $f_o=30cm$ and $f_e=6cm$

$M=5$

$f_o=?f_e=?$

$L=f_o+f_e=36cm$

As $M_{\infty }=\frac{f_o}{f_e}$

Therefore, $5f_e=f_o$

So, $6f_e=36cm$

$f_e=6cm$

Hence $f_o=5\times 6=30cm$