Question
A spherical mirror forms an erect image three times the linear size of the object. If the distance between the object and the image is $80\,cm$, the focal length of the mirror is........
✓
Answer
$\mathrm{u}=-\mathrm{x}$
$\mathrm{v}=3 \mathrm{x}$
$3 x+x+80$
$4 x=80 \Rightarrow x=20$
$\mathrm{u}=-20 \quad \mathrm{v}=60$
$\frac{1}{v}+\frac{1}{u}=\frac{1}{f}$
$\frac{1}{60}-\frac{1}{20}=\frac{1}{f}$
$f=-30 \mathrm{\,cm}$
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