First slide

The optical instruments

Question

If the focal length of objective and eye lens are 1.2 cm and 3 cm respectively and the object is put 1.25cm away from the objective lens and the final image is formed at infinity. The magnifying power of the microscope is

Moderate

A

150
B

200
C

250
D

400

Solution

$\begin{array}{l} m_{\infty} = \frac{v_{0}}{u_{0}} \times \frac{D}{f_{e}} \\ F r o m \frac{1}{f_{0}} = \frac{1}{v_{0}} - \frac{1}{u_{0}} \\ \Rightarrow \frac{1}{(+ 1.2)} = \frac{1}{v_{0}} - \frac{1}{(- 1.25)} \\ \Rightarrow υ_{0} = 30 \\ ∴ | m_{\infty} | = \frac{30}{1.25} \times \frac{25}{3} = 200 \end{array}$

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