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
Concept / Approach: Two equations, two unknowns. Solve.
\( M = \frac{f_o}{f_e} \)
\(L=f_o+f_e \)
\( \frac{f_o}{f_e}= 10 \)
\(f_o + f_e = 44 \)
\( \left(10\times f_e \right)+f_e = 44 \)
\(f_e = 4 \, \text{cm}\)
\( f_o = 40 \, \text{cm} \)
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