As described by Healthline.com, the cornea's main function is to refract light entering the eye. St. Luke's Cataract & Laser Institute adds that the refraction of the cornea provides two-thirds of the human eye's focusing power.
The National Eye Institute adds that the cornea is the eye's outermost window and that it controls as well as focuses any light coming in. In terms of controlling light, the cornea is able to screen out ultraviolet light in order to protect the rest of the eye.
The cornea has a secondary protective function, according to the National Eye Institute. It also serves to physically protect the eye by shielding it from germs and small particles. In this function, it teams with the eyelids, eye socket, sclera and eyelashes, which also provide physical barriers, and with tears, which wash away debris. The cornea's protective function may be secondary to its first role as a refractive surface, but it truly is key. Healthline.com notes that the cornea is located over the pupil, iris, and the anterior chamber. All of these are very delicate components which are vitally important to sight. The cornea is able to recover quickly from minor scratches, but deeper scars can cause the cornea to lose transparency and impede vision.