First slide

The optical instruments

Question

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

Moderate

A

$45 cm$
B

$55 cm$
C

$\frac{275}{6} cm$
D

$\frac{325}{6} cm$

Solution

Length of the telescope when final image is formed at least distance of distinct vision is
$L = f_{o} + u_{e} = f_{o} + \frac{f_{e} D}{f_{e} + D} = 50 + \frac{5 \times 25}{5 + 25} = \frac{325}{6} cm$

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