In the eyeball, light rays passing through the cornea are bent by its curvature toward the pupil. The lens flexes to change its curvature and finish the focusing process. Interestingly, the image projected on the retina is upside down because of the way the rays of light are bent by a double-convex lens. On the retina are two kinds of cells that change light into nerve impulses. Rod cells do not see color but are best for night viewing because they react to very low light levels.
Cone cells are for color viewing.
Within each eye is a small blind spot with no rods or cones, where the optic nerve is attached to the eyeball. The optic nerve collects the nerve impulses and carries them to the brain, which interprets them as an image. Many animals besides humans use vision. The eye and seeing. In the Human Body series.