71. The magnifying power of a telescope is 9 . When it is adjusted for parallel rays the distance between the objective and eyepiece is 20 cm. The focal length of lenses are :

71. The magnifying power of a telescope is 9 . When it is adjusted for parallel rays the distance between the objective and eyepiece is 20 cm. The focal length of lenses are :

(a) 10 cm, 10 cm

(b) 15 cm, 5 cm

(c) 18 cm, 2 cm

(d) 11 cm, 9 cm

Answer :  (c) 18 cm, 2 cm

Concept / Approach: Two equations, two unknowns. Solve.

\( M = \frac{f_o}{f_e} \)

\(L=f_o+f_e \)

\( \frac{f_o}{f_e}= 9 \)

\(f_o + f_e = 20 \)

\( \left(9\times f_e \right)+f_e = 20 \)

\(f_e = 2 \, \text{cm}\)

\( f_o = 18 \, \text{cm} \)