Which structure in the eye is responsible for focusing light onto the retina?

The lens is the primary structure in the eye responsible for focusing light onto the retina. It achieves this focus through a process called accommodation, where the lens changes shape to adjust the focus for objects at varying distances. When viewing something up close, the lens becomes more rounded, increasing its curvature to bend the light rays more sharply. Conversely, when looking at distant objects, the lens flattens out, which allows it to project the light onto the retina correctly.

Other structures, such as the cornea, do contribute to focusing light, as it is responsible for a significant portion of the eye's total refractive power. However, the lens provides fine-tuning adjustments to ensure that the light properly converges on the retina. The ciliary body plays a role in controlling the shape of the lens and facilitating accommodation, but it is not the structure that directly focuses the light. The iris controls the size of the pupil and regulates the amount of light entering the eye, but it does not focus light onto the retina. Thus, the lens is the correct answer, being the key player in the focusing process.