What happens to parallel light rays after passing through a convex lens?

• A. They converge to a focal point on the opposite side.
• B. They diverge away from the optical axis.
• C. They pass through undeviated.
• D. They reflect.

Answer: (A)

When parallel light rays strike a convex lens, the lens acts as a converging element. The curved surfaces refract the light so each ray bends toward the lens axis, and after passing through the lens they meet at a single point on the opposite side—the focal point. This focal point lies a distance equal to the lens’s focal length from the center. In the paraxial, or small-angle, region, all parallel rays are directed to pass through that focal point, producing convergence. The other possibilities would require a different optical behavior (a concave lens would cause divergence, undeviated passage would mean no refraction, and reflection involves bouncing light, not transmission through a lens). So parallel rays converge to the focal point on the far side.