Which lens corrects farsightedness?

• A. Concave
• B. Convex
• C. Plano-convex
• D. Cylindrical

Answer: (B)

Farsightedness happens when the eye’s optical system that focuses light is too weak, so parallel rays are brought to a focus behind the retina. To fix this, you need a lens that makes light converge more, moving the focal point forward onto the retina. A convex lens is a converging lens—the center is thicker and it bends light toward the axis, causing rays to meet sooner. That’s why a convex lens corrects farsightedness.

Concave lenses, by contrast, spread light apart and would push the focal point further back, worsening farsightedness. Cylindrical lenses address astigmatism by different amounts of focusing in different meridians, not by providing the necessary overall convergence. Plano-convex is a type of convex lens with a flat surface; it still provides convergence, but the essential effect that fixes farsightedness is the converging action of a convex lens.