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JEE Mains · Physics · STD 12 - Ray optics and optical instruments
  • JEE Mains 2019
  • Medium
  • MCQ

An object is at a distance of \(20\,m\) from a convex lens of focal length \(0.3\,m.\) The lens forms an image of the object. If the object moves away from the lens at a speed of \(5\,m/s,\) the speed and direction of the image will be

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Answer & Solution

Correct Answer

(D) \(1.16\times 10^{-3}\,m/s\) towards the lens

Step-by-step Solution

Detailed explanation

\(\frac{1}{V}-\frac{1}{-20}=\frac{1}{30}\)

\( \Rightarrow \frac{1}{V} = \frac{1}{{0.30}} - \frac{1}{{20}} = \) \( + \frac{{200 - 3}}{{60}} = \frac{{197}}{{60}}\)

\( \Rightarrow \quad  - \frac{{dV}}{{dt\,\,{V^2}}} + \frac{{du}}{{dt\,{u^2}}} = 0\)

\( \Rightarrow \frac{{dV}}{{dt}} = \frac{{{V^2}}}{{{u^2}}}\frac{{du}}{{dt}}\) \( = {\left( {\frac{3}{{197}}} \right)^2} \times ( - 5)\) \( =  - 0.00116\,{\text{m}}/{\text{s}}\)

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