CBSE Class 10 Science 2020 · Paper 31/5/3 · Question 29(a)

(a) From the top of the object, a ray parallel to the principal axis is refracted through $F_2$, and a ray through the optical centre O goes on undeviated. The refracted rays diverge; their dashed backward extensions meet on the object's side of the lens, where the dashed image A′B′ is drawn (see the completed diagram). (b) The image is virtual and erect; it is on the same side of the lens as the object, $180$ cm from the lens (beyond $2F_1$); it is enlarged, $10$ times the size of the object. (c) With $u=-18$ cm and $f=+20$ cm, $\frac{1}{v}=\frac{1}{f}+\frac{1}{u}=\frac{1}{20}-\frac{1}{18}=-\frac{1}{180}$, so $v=-180$ cm, and the magnification is $m=\frac{v}{u}=\frac{-180}{-18}=+10$.