What is true about the central ray (the one that passes through the center of the lens) in a typical ray diagram?

• A. It passes straight through without bending.
• B. It bends toward the focal point.
• C. It goes through the focal point on the opposite side.
• D. It travels parallel to the principal axis.

Answer: (A)

In lens ray diagrams, the central ray is drawn as passing straight through the optical center without bending. This happens because, at the center, the entry and exit surfaces of the lens bend the ray by equal and opposite amounts, so the net deviation is zero in the thin-lens approximation. This undeviated path makes the central ray a reliable reference for locating the image, while other rays—such as one parallel to the principal axis that refracts through the focal point, or one through the focal point that exits parallel to the axis—show the lens’s focusing behavior. So the central ray goes through the lens without changing direction.