Which statement describes how parallel light behaves when it passes through a converging lens?

• A. At the focal point on the opposite side
• B. At the focal point on the same side
• C. They diverge away from the focal point
• D. They never converge

Answer: (A)

When light comes from a distant object, it is effectively parallel. A converging (convex) lens bends those parallel rays so that they all meet at a single point on the opposite side of the lens, at the distance equal to the lens’s focal length. In other words, parallel rays are focused to the focal point on the far side of the lens. This follows the idea that for rays entering parallel to the axis, the lens directs them through the focal point on the opposite side (and, in the thin-lens model, the lens equation 1/f = 1/v − 1/u gives v = f when u is infinite).