CBSE Class 10 Science 2017 · Paper 31/1/1 · Question 23

(a) Two convenient rays: (i) a ray parallel to the principal axis, which after reflection from a concave mirror passes through the principal focus $F$; (ii) a ray passing through the centre of curvature $C$, which strikes the mirror along the normal and is reflected back along the same path. For an object placed between $P$ and $F$, the two reflected rays diverge; when produced backwards they meet behind the mirror. The image is formed behind the mirror and is virtual, erect and enlarged (see the ray diagram). (b) An image on a screen is real, hence inverted, so $m=-3$. With $u=-20$ cm, $m=-\frac{v}{u}$ gives $v=-60$ cm: the screen is $60$ cm in front of the mirror. The screen and the object are on the same side of the mirror, so the screen is $60-20=40$ cm from the object.