Which structure is essential for focusing light onto the retina?

The lens is essential for focusing light onto the retina because it has the ability to change its shape, a process known as accommodation. This adjustment allows the eye to focus on objects at various distances. When light enters the eye, it first passes through the cornea, which provides a significant portion of the eye's total refractive power due to its curvature. However, the cornea alone cannot adjust focus for objects at different distances.

Once the light passes through the cornea, it continues to the lens, which fine-tunes the focus. The lens can flatten or thicken depending on whether the object being viewed is far away or close up, ensuring that the light rays converge precisely on the retina to form a clear image. The retina then detects these light signals and converts them into neural signals for the brain to interpret.

The other structures listed, such as the pupil and iris, play important roles in regulating the amount of light entering the eye and controlling the size of the aperture for light entry but do not directly focus light on the retina. The iris controls the size of the pupil, which can limit or allow more light into the eye, but it does not contribute to the focusing process itself.