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

(a) (i) Pole: the centre of the reflecting surface of a spherical mirror; it lies on the surface of the mirror and is denoted by P. (ii) Centre of curvature: the centre of the hollow sphere of which the reflecting surface of the mirror is a part; it is denoted by C and is not a part of the mirror. (iii) Principal axis: the imaginary straight line passing through the pole and the centre of curvature of the mirror. (iv) Focal length: the distance between the pole and the principal focus of the mirror ($f=PF$). (b) $h=+4$ cm, $u=-15$ cm, $f=-10$ cm. Mirror formula: $\frac{1}{v}=\frac{1}{f}-\frac{1}{u}=-\frac{1}{10}+\frac{1}{15}=-\frac{1}{30}$, so $v=-30$ cm: the image is $30$ cm from the mirror, in front of it. Size of the image: $m=-\frac{v}{u}=-\frac{(-30)}{(-15)}=-2$, so $h^{\prime }=m\times h=-8$ cm: the image is $8$ cm tall, real and inverted (twice the size of the object).