Which structure provides most of the eye's focusing power by refracting incoming light?

• A. Lens
• B. Cornea
• C. Pupil
• D. Aqueous humor

Answer: (B)

The part that provides most of the eye's focusing power by refracting light is the cornea. The cornea is the clear, curved front surface of the eye, and the air-to-cornea interface bends a large portion of incoming light right at the outset. Its strong curvature and the large difference in refractive index between air and the corneal tissue produce most of the eye’s refractive power, roughly two-thirds of it. The lens behind the pupil also bends light and adds fine-tuning through accommodation, but it contributes less overall power than the cornea. The pupil itself is simply an opening that controls light entry; it does not bend light. The aqueous humor fills the anterior chamber and has only a minor role in refraction compared with the cornea.