How do the cornea and crystalline lens focus light on the retina?

The human eye functions as a complex optical system that focuses light onto the retina through refraction at four primary surfaces: the anterior and posterior surfaces of the cornea and the crystalline lens.1

When light enters the eye, the greatest degree of bending occurs at the air-cornea interface due to the significant difference in refractive index. The cornea provides the majority of the eye's fixed refractive power, while the lens provides additional convergence.1

To maintain focus on objects at varying distances, the eye utilizes a process called accommodation. When viewing near objects, the ciliary muscle contracts, which slackens the zonular fibres. This release of tension allows the elastic lens to become thicker and more convex, thereby increasing its refractive power to bring the image into focus on the retina. Conversely, when viewing distant objects, the ciliary muscle relaxes, pulling the zonular fibres taut and flattening the lens to decrease its power.234