Question

The magnifying power of terrestrial telescope is 25, when it is in normal adjustment and the length of the telescope is 124 cm. If the focal length of the errecting lens is 5 cm, the focal lengths of the objective and the eye-piece are respectively:

• A. 50cm, 2cm
• B. 50cm, 2.5cm
• C. 100cm, 4cm
• D. 100cm, 5cm

Answer 1

The correct option is C 100cm, 4cm

The magnification of a terrestrial telescope is given as, $M=$ $\frac{f_o}{f_e}$

Also, the length of a telescope is equal to, $L=$$f_o+f_e+4f$

Using $M=25,L=124$ and $f=5$
We get, $f_o=100$ and $f_e=4$