An object and its real image are located at distances 25 cm and 40 cm 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
Correct Answer:
B
Solution :
In the figure \[u=-(25+f)\] \[v=(40+f)\] using \[\frac{1}{v}-\frac{1}{u}=\frac{1}{f}\] \[\frac{1}{f+40}=\frac{1}{25+f}=\frac{1}{f}\] \[65f+2f=1000+65\,f+{{f}^{2}}\] \[{{f}^{2}}=1000\] \[f=31.62\,cm\] \[u=25+31.62\] \[=56.62\,cm\] \[v=40+31.62=71.62\,cm\] \[m=\frac{v}{u}=\frac{71.62}{-56.62}=-1.3\]