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  4. A student plots a graph between the inverse of magnification (1/m) produced by a thin convex lens and the object position u as shown in the figure. The focal length of the lens used, was <img class=img-fluid question-image alt=Question src=https://cdn.tardigrade.in/q/nta/p-0sx94l0jyqfb.png />

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Q. A student plots a graph between the inverse of magnification $\frac{1}{m}$ produced by a thin convex lens and the object position $u$ as shown in the figure. The focal length of the lens used, was

Question

NTA AbhyasNTA Abhyas 2020Ray Optics and Optical Instruments

Solution:

$\frac{1}{v}-\frac{1}{u}=\frac{1}{f}$
$\frac{u}{v}-1=\frac{u}{f}$
$\frac{u}{v}=\frac{u}{f}+1$
$\frac{1}{m}=\frac{u}{f}+1$
$slope=\frac{1}{f}$
$\frac{1}{f}=\frac{b}{c}$
$f=\frac{c}{b}$