A ray aimed at the focal point on the object side of a convex lens emerges on the other side parallel to the axis. This statement is?

• A. Sometimes true depending on lens design
• B. False
• C. True
• D. Only for objects at infinity

Answer: (C)

A convex lens follows a simple ray-tracing rule: a ray that travels toward the front focal point on the object side leaves the lens traveling parallel to the optical axis on the image side. This happens because the lens refracts the ray just enough so that, after passing through the lens, its direction is along the axis, i.e., parallel to it. This is a standard result in the thin-lens (paraxial) approximation, and it holds regardless of where the ray hits the lens—as long as it passes through the front focal point. In other words, directing a ray through the focal point on the object side makes it emerge parallel to the axis on the other side. Minor deviations can occur with thick lenses or large angles, but within the common model the statement is true.