68. An astronomical telescope has a length of 44 cm and tenfold magnification. The focal length of the objective lens is ?
(a) 4 cm
(b) 44 cm
(c) 40 cm
(d) 440 cm
Answer : (c) 40 cm
\( M = \frac{f_o}{f_e} = 10 \)
\( f_e = 44 - f_o \)
\( f_o = 440 - 10f_o \)
\( 11f_o = 440 \)
\( f_o = 40 \, \text{cm} \)
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