Question

An object and its real image are located at distances $25cm$ and $40cm$ respectively from the two principal foci of a convex lens. The linear magnification of the image is near to

• A. 1.3
• B. -1.3
• C. 1.8
• D. -1.8

Answer 1

Answer: (B)

In the figure
$u=-(25+f)$
$v=(40+f)$
using $\frac{1}{v}-\frac{1}{u}=\frac{1}{f}$
$\frac{1}{f+40}-\frac{1}{25+f}=\frac{1}{f}$
$65f+2f=1000+65f+f^2$
$f^2=1000$
$f=31.62cm$
$u=25+31.62=56.62cm$
$v=40+31.62=71.62cm$
$m=\frac{v}{u}=\frac{71.62}{-56.62}=-1.3$