One's eyes are just one organ in the system of organs that allows humans to see. The light first passes through the cornea then the lens. The lens and to a lesser extent, the cornea, refract the light, directing it to a point through the eye's vitreous fluid to land on the retina. The brain eventually associates the processed signals into known objects, such as one's computer monitor, letters, people, their faces, their nose, eyes, mouth. The clearest part of a human's vision is that that falls on the fovea. When one focuses on something, one is 'casting the image of that thing on one's fovea'.