Question

An astronomical telescope of tenfold angular magnification has a length of 44 cm. The focal length of the objective is

• A. 44 cm
• B. 440 cm
• C. 4 cm
• D. 40 cm.

Answer 1

Answer: (D)

40 cm.

Answer 2

Length of astronomical telescope
$(f_0 + f_e ) = 44 cm$ and ratio of focal length of the
objective lens to that of the eye piece $\frac{f_0}{f_e} = 10$
From the given ratio, we find that $f_0 =10 f_e$
Therefore $10f_e +f_e = 44 \, \, or \, \, f_e = 4 cm$
and focal length of the objective $(f_0)$
$= 44- f_e = 44-4 =40 cm$