First slide

The optical instruments

Question

The magnifying power of a microscope with an objective of 5 mm focal length is 400. The length of its tube is 20 cm. Then the focal length of the eye-piece is

Moderate

A

200 cm
B

160 cm
C

2.5 cm
D

0.1 cm

Solution

If nothing is said then it is considered that final image is formed at infinite and $m_{\infty} = \frac{(L_{\infty} - f_{o} - f_{e}) . D}{f_{\infty} f_{e}} ≃ \frac{LD}{f_{0} f_{e}}$
$\Rightarrow 400 = \frac{20 \times 25}{0 .5 \times f_{e}} \Rightarrow f_{e} = 2 .5 cm$

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