The rectangular box is the place of a spherical mirror. The point S is the position of an object and the point I is the position of its image. The height of the point S from the axis is two times that of point I. The horizontal separation between S and I is equal to 15 cm.

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S and I are on the same side of the axis, so the image is erect and $m = + \frac{1}{2}$ .

Let object be at $x = - a$ .

Then image is at $+ \frac{a}{2}$ (behind the mirror). $a + \frac{a}{2} = 15 \Rightarrow a = 10 cm$ .

Object at -10 cm, image at +5 cm.

$\frac{1}{v} + \frac{1}{u} = \frac{1}{f} \Rightarrow \frac{1}{5} - \frac{1}{10} = \frac{1}{10}, so f = + 10 cm .$

(A) and (D) are wrong.

Ans. (B), (C)

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