CBSE Class 10 Science 2017 · Paper 31/2/1 · Question 23

(a)(i) Pole: the centre of the reflecting surface of a spherical mirror. It is a point on the surface of the mirror, usually written $P$. (a)(ii) Centre of curvature: the centre of the sphere of which the reflecting surface of the mirror is a part, written $C$. It is not a part of the mirror; it lies in front of a concave mirror and behind a convex mirror. (a)(iii) Radius of curvature: the radius of the sphere of which the reflecting surface of the mirror is a part, written $R$; it is equal to the distance $PC$. (a)(iv) Principal axis: the straight line passing through the pole and the centre of curvature of a spherical mirror; it is normal to the mirror at its pole. (b)(i) Concave mirror: rays parallel to the principal axis, after reflection, converge and actually pass through a point $F$ on the principal axis in front of the mirror. This is the principal focus; it is real. (b)(ii) Convex mirror: rays parallel to the principal axis, after reflection, diverge and appear to come from a point $F$ on the principal axis behind the mirror. This is the principal focus; it is virtual. For both mirrors of small aperture $F$ lies midway between $P$ and $C$ ($f=R/2$). (c) The object lies between $F$ and the pole. A ray from $A$ parallel to the principal axis is reflected through $F$, and a ray from $A$ along the line from $C$ is reflected back along itself. The reflected rays diverge; their backward extensions meet behind the mirror at $A^{\prime }$. The image $A^{\prime }{B}^{\prime }$ is formed behind the mirror and is virtual, erect and enlarged.