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  4. A spherical mirror forms diminished virtual image of magnification 1 / 3. Focal length is 18 cm. The distance of the object is

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Q. A spherical mirror forms diminished virtual image of magnification $1 / 3$. Focal length is $18\, cm$. The distance of the object is

Ray Optics and Optical Instruments

Solution:

Clearly, the given mirror is a convex mirror.
As $m=\frac{f}{f-u}$
$\Rightarrow \frac{1}{3}=\frac{18}{18-u}$
or $3 \times 18=18-u$
or $u=-2 \times 18 \,cm$
or $u=-36 \,cm$