A student measures the focal length of a convex lens by putting an… | JEE Main 2008 (AIEEE)

Q106JEE Main 2008 · AIEEEMedium

Spherical Surfaces & Thin Lenses

A student measures the focal length of a convex lens by putting an object pin at a distance $'u'$ from the lens and measuring the distance $'v'$ of the image pin. The graph between $'u'$ and $'v'$ plotted by the student should look like

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Answer: (c)

Real-is-positive convention: $u$ is negative and $v$ positive, so the graph lies in the second quadrant.
$\frac{1}{v} - \frac{1}{u} = \frac{1}{f}$ : as $|u| \to f$ , $v \to \infty$ ; as $|u| \to \infty$ , $v \to f$ .
This is a rectangular-hyperbola-like curve in the second quadrant.
So the answer is (c).

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Spherical Surfaces & Thin Lenses question from JEE Main 2008 (AIEEE)

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