Question

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

• A. $12$
• B. $11$
• C. $10$
• D. $13$

Answer 1

The correct option is A $12$

For final image formed at near point,

Magnification of the compound microscope

$m=m_0(1+\frac{D}{f_e})$

$m_0$ is the magnification of the objective lens given by
$m_0=\frac{f}{f+u}=\frac{4}{4-5}$
$=-4$

$1+\frac{D}{f_e}=1+\frac{20}{10}=3$

So net magnification $m=4\times 3=12$