Question

The graph shows how the reciprocal of magnification $\left(|\mu =\frac{4}{3}|\right)$ produced by a converging thin lens varied with object distance $u$ in displacement method. What was the focal length of the lens used?

• A. $a/b$
• B. $bc/a$
• C. $c/b$
• D. $ab/c$

Answer 1

Answer: (C)

$c/b$

By applying lens formula $\frac{1}{v}-\frac{1}{u}=\frac{1}{f}$

$\frac{1}{v}=\frac{f+u}{uf}$; $m=\frac{v}{u}$ $\Rightarrow$ $\frac{f}{f+u}$; $\frac{1}{m}=\frac{u}{f}+1$

Slope of $\frac{1}{m}$ and ${}^{\prime }{u}^{\prime }$ is $\frac{1}{f}$ $\therefore$ $\frac{1}{f}=\frac{b}{c}$ $\Rightarrow$ $f=\frac{c}{b}$