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  • At what distance from a convex lens of focal length an object should be placed, so that

At what distance from a convex lens of focal length \mathrm{30\ cm} an object should be placed, so that the size of image be \mathrm{1/4^{th}} of the object?

Option: 1

\mathrm{30\ cm}


Option: 2

\mathrm{60\ cm}


Option: 3

\mathrm{15\ cm}


Option: 4

\mathrm{150\ cm}


Answers (1)

best_answer

Image is real
\mathrm{\therefore \mathrm{m}=\frac{\mathrm{v}}{\mathrm{u}}=-\frac{1}{4} \ \ \ \ \ \quad \text { or } \quad \mathrm{v}=-\frac{\mathrm{u}}{4}}
\mathrm{\begin{aligned} & \text { As } \frac{1}{\mathrm{v}}-\frac{1}{\mathrm{u}}=\frac{1}{\mathrm{f}} \\ & \therefore \frac{1}{(-\mathrm{u} / 4)}-\frac{1}{\mathrm{u}}=\frac{1}{30} \end{aligned}}
\mathrm{-\frac{4}{\mathrm{u}}-\frac{1}{\mathrm{u}}=\frac{1}{30} \quad \text { or } \quad \mathrm{u}=-150 \mathrm{~cm}}

Posted by

Irshad Anwar

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