Which structure in the eyeball is composed of a transparent epithelium and helps focus light onto the retina?

The correct answer is the lens, which is the structure in the eyeball composed of a transparent epithelium and plays a crucial role in focusing light onto the retina.

The lens is a flexible, transparent structure located behind the iris that changes shape to adjust the focal distance. This ability to change shape is essential for focusing on objects at varying distances, a process known as accommodation. The transparency of the lens allows light to pass through without obstruction, enabling clear vision.

While the cornea also contributes to the refraction of light entering the eye, it is primarily the lens that fine-tunes focus, particularly for near and distant objects. The iris controls the amount of light that enters the eye but does not play a direct role in focusing. The sclera, on the other hand, is the white outer layer of the eyeball and provides structure, but it does not have any optical properties.