Explanation

(b) Object is placed at distance 2$f$ from the lens. So first image $I$ will be formed at distance 2$f$ on other side. This image $I_{1}$ will behave like a virtual object for mirror. The second image $I_{2}$ will be formed at distance 20 cm in front of the mirror, or at distance 10 cm to the left hand side of the lens.
Now applying lens formula
$\frac{1}{v} - \frac{1}{u} = \frac{1}{f}$
[image:9_image70.emf]
$\therefore\frac{1}{v} - \frac{1}{+ 10} = \frac{1}{+ 15}$
$or\ v = 16\ cm$
Therefore, the final image is at distance 16 cm from the mirror. But, this image will be real.
This is because ray of light is travelling from right to left.