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

The lens is the structure in the eye that plays a crucial role in focusing light onto the retina. It is a transparent, flexible structure located behind the iris and the pupil. The lens can change shape due to the action of the ciliary muscles surrounding it, allowing for adjustment of focus for objects at varying distances. This ability to change shape enables the lens to provide clear images on the retina, where light is converted into electrical signals sent to the brain.

The cornea also contributes to the eye's focusing power, as it has a significant curvature and is responsible for most of the eye's total refractive power. However, the lens is specifically responsible for the fine-tuning of focus, particularly when it comes to seeing objects at different distances.

The pupil is the opening in the center of the iris and regulates the amount of light that enters the eye, but it does not participate in the focusing process. The iris is the colored part of the eye that controls the size of the pupil, thus managing light intake but not focusing light itself. Therefore, the lens is the definitive answer for the structure responsible for focusing light directly onto the retina.