Question

The focal length 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. 45 cm
• B. 55 cm
• C. $\frac{275}{6}cm$
• D. $\frac{325}{6}cm$

Answer 1

Answer: (D)

$\frac{325}{6}cm$

Answer 2

: Here, $f_{o} = 50cm,f_{e} = 5cm,D = 25cm$

The length of the telescope when the image is formed at the least distance of distint vision is

$L = f_{o} + \frac{f_{e}D}{f_{e} + D} = 50 + \frac{5 \times 25}{5 + 25} = 50 + \frac{25}{6} = \frac{325}{6}cm$