In the concave mirror, a ray incident parallel to the principal axis is reflected ___.

Concave mirrors are used to collect light from light sources and to make images of them. When a beam of parallel rays strikes a perfect concave mirror, they are reflected and all pass through the same point. If a beam is parallel to the principal axis of a mirror, this point is the focal point. So the words to fill the blank are "in such a way that it goes through the focal point of a mirror".

The perfect shape of a concave mirror is a paraboloid of revolution. For such a mirror, parallel rays are actually go through a point. Concave spherical mirrors, usually used for simple tasks, exhibit spherical aberration: reflected rays go close, but not through one point.

Mirrors have some advantages over lens. They are much lighter and do not introduce chromatic aberrations.