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
\( M = \frac{f_o}{f_e} \)
\( M = 9 \)
\( f_e = 20 - f_o \)
\( 9 = \frac{f_o}{20 - f_o} \)
\( 180 - 9f_o = f_o \)
\( 180 = 10f_o \)
\( f_o = 18 \, \text{cm} \)
\( f_e = 2 \, \text{cm} \)
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