Which structure in the eye provides most of the eye's optical power?

The cornea is the structure in the eye that provides most of the eye's optical power because it has a fixed curvature and is the primary refractive surface. When light enters the eye, it first passes through the cornea, which bends (or refracts) the light rays significantly due to the difference in density between air and the corneal tissue. This bending is crucial for focusing light onto the retina at the back of the eye, which is necessary for clear vision.

While the lens also contributes to the eye's refractive ability by changing shape to fine-tune focus for near and far objects, its overall optical power is less than that of the cornea due to its smaller curvature and the fact that it predominantly adjusts focus rather than providing the initial refractive power. The iris, on the other hand, is primarily involved in controlling the amount of light entering the eye and does not contribute to refraction. The retina is responsible for detecting light and sending visual signals to the brain, but it does not play a role in focusing the light itself. Thus, the cornea stands out as the main contributor to the eye's optical power.