For a concave mirror, like these pins in fig.-1(b), all normals will converge towards a point. This point is called centre of curvature(C) of the mirror.

Recall a little bit of geometry: while learning about circles and tangents, you have learnt that a radius is always perpendicular to the tangent to the circle drawn at the point.

This gives us a clue about how we can find normal to any point on a spherical mirror. All that we have to do is to draw a line from the point on the mirror to centre of the sphere.

It is much easier to imagine this in a two dimensional fig. as shown in fig.-2(a). The concave mirror is actually a part of a big sphere. In order to find this centre point (centre of curvature) we have to think of the centre of the sphere to which the concave mirror belongs. The line drawn from C to any point on the mirror gives the normal at that point.

For the ray R, the incident angle is the angle it makes with the normal shown as i and the reflected angle is shown as r in fig.-2(b). We know by first law of reflection i = r.

The mid point (Geometrical centre) of the mirror is called pole (P) of the mirror. The horizontal line shown in the fig.s which passes through the centre of curvature and pole is called principal axis of the mirror. The distance between P and C is radius of curvature (R) of the mirror. Try to construct different reflected rays for any array of rays that are parallel to the principal axis as shown in fig. 2(b). What is your conclusion?

Verifying your drawing with experiment: