Question

An astronomical telescope has on objective of focal length $200cm$ and an eye piece of focal length $4.0cm$. The telescope is focused to see an object $10km$ from the objective. The final image is formed at infinity. Find the length of the tube and the angular magnification produced by the telescope.

• A. $196cm,-50$
• B. $204cm,-50$
• C. $206cm,-50$
• D. $200cm,-50$

Answer 1

The correct option is B $204cm,-50$

Here, the object is at a large distance $\left(10km\right)$ compared to the focal length of the objective. Hence the rays coming from the object can be treated as parallel.

Length of the tube $L=f_0+f_e=200+4=204cm$

Angular magnification $m=\frac{-f_0}{f_e}=\frac{200}{4}=-50$

The final image will be real inverted.