What is the role of the cornea in the eye?

• A. Provides ~⅔ of total eye refraction and determines focal length
• B. Fine-tunes focus by altering curvature for near objects
• C. Converts focused photons into graded potentials and action potentials
• D. Modulates aperture size to improve depth of field

Answer: (A)

Light entering the eye is bent mainly at the air–cornea boundary, giving the cornea most of the eye’s focusing power. Because this interface involves a large change in refractive index and the cornea has a steep, curved surface, it contributes roughly two-thirds of the total optical power—setting the primary focal length of the eye. The remaining focusing power comes from the crystalline lens, which can adjust focus for near versus far objects through accommodation. The cornea’s curvature is relatively fixed in daily vision, so shifting focus to near objects relies on the lens changing shape, not the cornea. Photons are converted into electrical signals by retinal photoreceptors, not by the cornea. Depth of field and light control are influenced by pupil size, which the iris modulates as an aperture.