CBSE Class 10 Science 2018 · Paper 31(B)/C · Question 20(b)

(a) Laws of reflection: (i) The angle of incidence is equal to the angle of reflection. (ii) The incident ray, the normal to the mirror at the point of incidence and the reflected ray all lie in the same plane. (b) Any two: the image formed by a plane mirror is virtual and erect; it is of the same size as the object; it is as far behind the mirror as the object is in front of it; it is laterally inverted. (c) The mirror is concave ($f=-20$ cm) and $u=-30$ cm. By the mirror formula, $\frac{1}{v}=\frac{1}{f}-\frac{1}{u}=-\frac{1}{20}+\frac{1}{30}=-\frac{1}{60}$, so $v=-60$ cm. The screen should be placed $60$ cm from the mirror, on the same side as the object (in front of the mirror). The image on it is real, inverted and twice the size of the object ($m=-2$).