What is the function of the lens in the eye?

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

Answer: (A)

The lens mainly functions to adjust focus for near objects by changing its curvature (accommodation). When you look at something close, the ciliary muscle contracts, loosening the zonular fibers and allowing the lens to become thicker and more curved. This increases the lens’s refractive power, shortening the focal length so light rays converge properly onto the retina. For distant objects, the ciliary muscle relaxes, the lens flattens, and the focal length lengthens to keep the image in focus.

The other ideas don’t fit this role: the cornea provides most of the eye’s refraction, while the lens adds dynamic focusing rather than a fixed power; photons are converted into electrical signals by photoreceptors (rods and cones) in the retina, not by the lens; and the pupil’s size (aperture) is controlled by the iris, not by the lens.