The point on the principal axis that all light rays travel through.

• A. Center
• B. Focus
• C. Vertex
• D. Point

Answer: (B)

The focus is the point on the principal axis where parallel light rays converge after passing through a converging lens or reflecting from a concave mirror. This single point is the location where the image of distant objects is formed, which is why it’s described as where rays “meet” on the axis. It’s different from the center (the center of curvature) and from the vertex (the point where the surface meets the axis), and simply calling it a generic point doesn’t capture that special role. The idea you might’ve seen—that light goes through a single point on the axis—fits best with the focus, because that’s the location described by how these optical elements bend or reflect light to bring rays to a common intersection.