The magnification of an astronomical telescope is 10 and the focal length of eye-piece is 20 cm. The focal length of objective lens is :

Question

The magnification of an astronomical telescope is 10 and the focal length of eye-piece is 20 cm. The focal length of objective lens is : (A)  (1/200) cm (B) 2 cm (C) 200 cm (D) 100 cm

Tardigrade Admin · Accepted Answer

Magnification of astronomical telescope m=(f0/20) where fo= focal length of objective lens fe= focal length of eye piece ∴ 10=(fo/20) ⇒ fo=10× 20 ⇒ fo=200 cm

Q. The magnification of an astronomical telescope is 10 and the focal length of eye-piece is 20 cm. The focal length of objective lens is :

$\frac{1}{200}$ cm

2 cm

200 cm

100 cm

Magnification of astronomical telescope $m = \frac{f _{0}}{20}$ where $f_{o} =$ focal length of objective lens $f_{e} =$ focal length of eye piece $∴$ $10 = \frac{f _{o}}{20}$ $\Rightarrow$ $f_{o} = 10 \times 20$ $\Rightarrow$ $f_{o} = 200 c m$