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

(a) 4 cm

(b) 40 cm

(c) 44 cm

(d) 440 cm

(b)

Given: Length of astronomical telescope (f₀ + f_e) = 44 cm and magnification

 (f_0/f_e )=10.

From the given magnification, we find that f₀ = 10 f_e.

Therefore, 10 f_e + f_e = 44 or 11 f_e = 44 or f_e = 4.

And focal length of the objective (f₀) = 44 - f_e = 44 - 4 = 40 cm.