Q. The focal lengths of the lenses of an astronomical telescope are $50\ cm$ and $5\ cm$. The length of the telescope when the image is formed at the least distance of distinct vision is
A) $45cm$
B) $55\ cm$
C) $\frac{275}{6}cm$
D) $\frac{325}{6}cm$
Explanation
(d) Length of the telescope when final image is formed at least distance of distinct vision is
$L = f_{o} + u_{e} = f_{o} + \frac{f_{e}D}{f_{e} + D} = 50 + \frac{5 \times 25}{5 + 25} = \frac{325}{6}cm$
Reviewed by Dr. Abubacker
Physics Educator & NEET/JEE Specialist
Ph.D. in Physics with 19+ years of teaching experience.