CBSE Class 10 Science 2017 · Paper 31/2/3 · Question 16

The object is 30 cm from the optical centre ($u=-30$ cm), with the image 30 cm from the lens on the other side ($v=+30$ cm); the object is at $2F_1$. The lens is a converging (convex) lens of focal length $f=+15$ cm. When the object is displaced 20 cm towards the optical centre, it is 10 cm from the lens ($u=-10$ cm), between $F_1$ and $O$. The image is then formed 30 cm from the lens on the same side as the object ($v=-30$ cm); it is virtual, erect and magnified, three times the size of the object ($m=+3$). In the ray diagram the two refracted rays diverge, and their backward extensions meet at that point.