Question

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

• A. $200cm$
• B. $160cm$
• C. $2.5cm$
• D. $0.1cm$

Answer 1

Answer: (C)

$2.5cm$

If nothing is said then it is considered that final image if formed at infinite and $m_{\infty }=\frac{(L_{\infty }-f_0-f_e).D}{f_0{f}_e}\simeq \frac{LD}{f_0{f}_e}$
$\Rightarrow 400=\frac{20\times 25}{0.5\times f_e}\Rightarrow f_e=2.5cm$.