Question

The linear magnification $m$ for the image formed at the near point i.e., $25cm$ is

• A. $\left(1-\frac{v}{f}\right)$
• B. $\left(1+\frac{d}{f}\right)$
• C. $\left(1+\frac{v}{f}\right)$
• D. $\left(1-\frac{d}{f}\right)$

Answer 1

Answer: (B)

The linear magnification $m$, for the image formed at the near point $d$, by a simple microscope can be obtained by using the relation.
$m=\frac{v}{u}=v\left(\frac{1}{v}-\frac{1}{f}\right)=1-\frac{v}{f}$
Now, according to our sign convention, $v$ is negative, and is equal in magnitude to $d$. Thus, the magnification is
$m=\left(1+\frac{d}{f}\right)$
Since, $d$ is about $25cm$, to have magnification of six, one needs a convex lens of focal length, $f=5cm$.