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The image of a small electric bulb fixed on the wall of a room is to be obtained on the opposite wall $3 \mathrm{~m}$ away by means of a large convex lens. What is the maximum possible focal length of the lens required for the purpose?
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Verified Answer
If $\mathrm{d}$ be the distance between object and wall then for real image on the wall $\mathrm{f}$ should not be greater than $\frac{\mathrm{d}}{4}$.
$$
\therefore \mathrm{f}_{\max }=\frac{\mathrm{d}}{4}=\frac{3}{4}=0.75 \mathrm{~m}
$$
$$
\therefore \mathrm{f}_{\max }=\frac{\mathrm{d}}{4}=\frac{3}{4}=0.75 \mathrm{~m}
$$
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