Which part of the eye has the greatest dioptric power?

The cornea has the greatest dioptric power among the structures of the eye. Dioptric power refers to the ability of a lens to bend light, which is measured in diopters. The cornea contributes the most to the eye's total refractive power due to its curved shape and the significant difference in refractive index between air and the corneal tissue.

While the lens also has dioptric power and can change its shape to focus on objects at varying distances (accommodation), the cornea remains responsible for the majority of light bending as it is the first major refracting surface that light encounters. The corneal surface is also more curved than the lens's surface, which enhances its ability to refract light.

The iris, primarily involved in controlling the amount of light entering the eye by adjusting the size of the pupil, does not contribute to dioptric power. It plays a role in regulating light but is not a refractive structure. Likewise, the retina is crucial for converting light images into neural signals but does not bend light or have dioptric power. Therefore, while the lens and other structures play important roles in vision, it's the cornea that has the greatest dioptric power.