The cornea, smooth and clear as glass, strong and durable, helps the eye by shielding it from germs, dust and other harmful matter. It acts like a window that controls and focuses the entry of light into the eye. Light that strikes the healthy cornea refracts onto the lens which further refocuses that light onto the retina. The retina then converts the light rays into impulses that are sent through the optic nerve to the brain, which interprets them as images.