The two convenient rays, chosen to construct the image by a spherical mirror are —
(i) A ray that passes through the centre of curvature.
A line joining the centre of curvature to any point on the surface of the mirror is normal to the mirror at that point, therefore a ray AD passing through the centre of curvature C (or appearing to pass through through the centre of curvature C) is incident normally on the spherical mirror.
Since it’s angle of incidence is zero, therefore the angle of reflection will also be zero and the ray AD gets reflected along it’s own path DA as shown below:
(ii) A ray parallel to the principal axis.
A ray of light AD incident parallel to the principal axis, after reflection passes either through the focus F(in a concave mirror) or will appear to come from the focus F (in a convex mirror) along DB as shown below:
