Question

at which light rays parallel to the principal axis"

Answer 1

The light rays that are parallel to the principal axis get reflected such that they pass through the focal point of the mirror

Answer 2

Explanation:

As we know that when a ray passes through center of curvature it falls back. It happens as it is a centre of sphere , which act as normal to circumference (concave mirror).

So when a ray incident parallel to principal axis it passes through focus because here the normal for the incident ray and reflected ray is the line connecting the point of incidence( CA): figure:; and the center of curvature so, according the law of reflection the angle of incidence is = angle of reflection