Question

The graph shows how the magnification m produced by a convex thin lens varies with image distance v. What was the focal length of the used

• A. $\frac{b}{c}$
• B. $\frac{b}{ca}$
• C. $\frac{bc}{a}$
• D. $\frac{c}{b}$

Answer 1

The correct option is D $\frac{c}{b}$

For a lens $m=\frac{f-v}{f}=-\frac{1}{f}$
Comparing it with y = mx+c
Slope = $m=-\frac{1}{f}$
From graph, slope of the line = $\frac{b}{c}$
Hence $-\frac{1}{f}=\frac{b}{c}\Rightarrow |f|=\frac{c}{b}$.