Question

A telescope has an objective of focal length $50\, cm$ and an eye-piece of focal length $5\, cm$. The least distance of distinct vision is $25\, cm$. The telescope is focussed for distinct vision on a scale $200\, cm$ away. The separation between the objective and the eye-piece is

• A. 75 cm
• B. 60 cm
• C. 71 cm
• D. 74 cm

Answer 1

Answer: (C)

71 cm

Answer 2

$\frac{1}{v_{o}}+\frac{1}{200}=\frac{2}{50}$ $\Rightarrow v_{0}=\frac{200}{3}=66.6\, cm$ $\therefore L=v_{o}+f_{e}=66.6+5 \approx 71\, cm$