Question

Find the position of an object when placed in front of a concave mirror of a focal length 20 cm produces a virtual image twice the size of the object.

Answer 1

Given f=-20cm m=+2 because the image is virtual and erect.

$-\frac{v}{u}=2$ $v=-2u$

$\frac{1}{f}=\frac{1}{u}+\frac{1}{v}$

$\frac{1}{-20}=\frac{1}{u}-\frac{1}{2u}$

$\frac{1}{-20}=\frac{1}{2u}$

$2u=-20$

$u=-10cm$

so the object is placed at a distance of 10cm from the mirror.