Which part of the eye provides two-thirds of the focusing power of the eye?

The cornea is responsible for providing approximately two-thirds of the eye's total focusing power, primarily due to its curved shape and refractive index, which bend light rays entering the eye. This refraction is crucial for bringing light to a focus on the retina, which is essential for clear vision. The cornea's fixed shape means that its contribution to focusing is more significant than that of the lens, which provides the remaining third by fine-tuning the focus through accommodation—adjusting the curvature of the lens for near or far vision.

Understanding the role of the cornea is fundamental in ophthalmic practice, as it plays a pivotal role in refractive errors and the overall optical system of the eye. The other structures mentioned, such as the iris, lens, and retina, serve different functions in vision. The iris regulates the amount of light entering the eye, the lens adjusts focus, and the retina converts light into neural signals for the brain.