Final answer:
Concave lenses are diverging lenses that cause parallel light rays to spread apart, creating the illusion that they emanate from a virtual focal point, which leads to a negative focal length and prevents the rays from coming to a real focus.
Step-by-step explanation:
The reason concave lenses don't bring light rays into focus is because they are diverging lenses. Unlike convex lenses, which have a positive focal length and bring light rays together at a focal point, concave lenses cause light rays to spread apart. When parallel light rays enter a concave lens, they bend away from the axis and diverge as if they were emanating from a focal point on the same side of the lens as the light source. This point is virtual, meaning it cannot be projected onto a screen, because the light rays never actually meet but appear to diverge from that point, resulting in a negative focal length for the lens.