What structure in the eye completes the focusing of light?

The lens is the structure in the eye that plays a crucial role in completing the focusing of light onto the retina. Positioned behind the iris and the pupil, the lens can change its shape through the action of ciliary muscles, allowing it to focus light from objects at various distances. This process is called accommodation. When we look at near objects, the lens becomes more rounded to increase its refractive power, while it flattens for distant objects.

The cornea, which is the eye's outermost layer, provides the majority of the eye's total optical power through refraction, but it doesn't adjust its shape for focusing. The iris primarily controls the size of the pupil and hence the amount of light that enters the eye, but it is not involved in focusing light. The fovea is a small depression in the retina where visual acuity is highest, as it is heavily populated with cone photoreceptors, but it does not have a role in focusing light itself. Hence, the lens is essential for fine-tuning the focus and is central to the eye's ability to present a clear image to the brain.