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  4. A students plots a graph between inverse of magnification 1/m produced by a convex thin lens and the object distance u as shown in figure. What was the focal length of the lens used?

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Q. A students plots a graph between inverse of magnification 1/$m$ produced by a convex thin lens and the object distance $u$ as shown in figure. What was the focal length of the lens used?Physics Question Image

VITEEEVITEEE 2013Ray Optics and Optical Instruments

Solution:

Lens formula,$\frac{1}{v}-\frac{1}{u}=\frac{1}{f}$
or $\frac{u}{v}-1=\frac{u}{f}$
$\Rightarrow \frac{1}{m}-1=\frac{u}{f}$
or $ \frac{1}{m}\left(\frac{1}{f}\right)u+1$
this is the equation of a straight line whose slope
$\frac{1}{f}=\frac{b}{c} $
$\therefore f = \frac{c}{b}$