Which eye structure is primarily involved in focusing light?

The correct answer is the lens, which plays a critical role in focusing light as it enters the eye. The lens is a flexible, transparent structure located behind the iris and the pupil. It changes shape to adjust the focal distance, allowing us to see objects clearly at various distances. This process is known as accommodation.

While the cornea also contributes to the eye's focusing power by bending light as it enters, its curvature is fixed and does not change shape the way the lens does. The iris and pupil are primarily involved in regulating the amount of light that enters the eye rather than focusing it. The iris controls the size of the pupil, which adjusts to varying light conditions, but neither structure has the ability to change focal lengths like the lens. Thus, the lens is the correct choice for the structure primarily responsible for focusing light.