Which structure first intercepts light that enters the eye?

The cornea is the first structure that intercepts light entering the eye. This transparent, dome-shaped surface covers the front part of the eye and plays a crucial role in focusing light onto the retina. When light rays enter the eye, they first pass through the cornea, which bends or refracts these rays toward the lens for further focusing. The cornea provides nearly 75% of the eye's total optical power due to its curvature and refractive properties.

The other structures, such as rods and cones, are photoreceptor cells located in the retina, responsible for converting light into electrical signals that the brain interprets as visual images. The iris, on the other hand, is the colored part of the eye that controls the size of the pupil and thus the amount of light entering the eye, but it is not the first structure to intercept light. The correct answer emphasizes the critical role of the cornea in the initial step of vision.