When a ray of light passes through a concave lens parallel to its principal axis, it diverges as if coming from a virtual focus point behind the lens. The concave lens is thinner at the center than at the edges, causing the light to refract outward. Unlike a converging convex lens, a concave lens spreads the incoming parallel rays, creating a diverging effect. The virtual focus point is the point where the refracted rays appear to originate when extended backward. This behavior is a characteristic of concave lenses and is vital in understanding their role in vision correction and optical systems.