Which part of the eye is responsible for focusing light?

The lens of the eye 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 changes its shape to adjust the focal distance, allowing the eye to focus on objects at various distances—a process known as accommodation. When looking at distant objects, the lens flattens, and when focusing on closer objects, it becomes more rounded, increasing its refractive power. This ability to change shape is facilitated by the ciliary muscles, which contract or relax to alter the lens’ curvature.

The cornea does contribute to light focusing, but it is primarily responsible for most of the eye's total refractive power, performing a significant portion of the initial light bending as it enters the eye. On the other hand, the iris controls the amount of light that enters the eye through the pupil but does not focus light. Lastly, the choroid layer is involved in nourishing the outer layers of the retina and absorbing stray light, but it does not focus light either. Thus, the lens is the key structure for the precise focusing of light onto the retina, making it the correct answer.