First slide

The optical instruments

Question

A compound microscope has an eye piece of focal length 10 cm and an objective of focal length 4 cm. Calculate the magnification, if an object is kept at a distance of 5 cm from the objective so that final image is formed at the least distance vision (20 cm)

Moderate

A

12
B

11
C

10
D

13

Solution

$For objective lens \frac{1}{f_{o}} = \frac{1}{v_{o}} - \frac{1}{u_{o}}$
$\Rightarrow \frac{1}{v_{o}} = \frac{1}{f_{o}} + \frac{1}{u_{o}} = \frac{1}{4} + \frac{1}{- 5} = \frac{1}{20} \Rightarrow v_{0} = 20 cm Now M = \frac{v_{o}}{u_{o}} (1 + \frac{D}{f_{e}}) = \frac{20}{5} (1 + \frac{20}{10}) = 12 .$

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