CBSE 2025 · Region 4 · Set 1 · Q31 · 3 marks
If we want to obtain a virtual and magnified image of an object by using a concave mirror of focal length $\displaystyle 18$ cm, where should the object be placed? Use mirror formula to determine the object distance for an image of magnification +$\displaystyle 2$ produced by this mirror to justify your answer.
Marking-scheme solution
Object should be placed between F and P / At less than 18cm distance from the mirror.
\[\text { Mirror formula }=\frac{1}{v}+\frac{1}{u}=\frac{1}{f}
\]
\[\begin{aligned}
\frac{1}{-2 u}+\frac{1}{u} & =\frac{1}{-18 c m} \\
\therefore \frac{1}{2 u} & =\frac{1}{-18 \mathrm{~cm}}
\end{aligned}
\]
\[\mathrm{U}=-9 \mathrm{~cm}
\]
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