Humans get the majority of sensory information from the visual system; light that strikes the eyes is converted to electrical impulses that are then interpreted by the brain. The visual system consists of the eyes, the optic nerve and the occipital lobe of the brain. Eyes are the most sensitive of all sensory organs, able to adjust quickly to changes in light and able to detect as many as 10 million colors. The ability to track the motions of objects and predict their future trajectory is another ability of human vision. Most animals lack motion vision because their retinas do not create afterimages.
The main working components of the eye are the iris, lens and retina. The eye has a hard cover of transparent material called the cornea. As light passes through the cornea and the pupil, the pupil's size changes with the amount of light available. The colored part of the eye, the iris, allows the pupil to dilate in lower light and contract in bright conditions. Images strike the lens, located just behind the pupil.
As light particles pass through the lens, the lens focuses them onto the retina. The convex shape of the lens flips the image upside-down as it strikes the retina. The human retina contains millions of photoreceptors that use specialized proteins to convert the image into electrical impulses, which are then processed by the brain. Two main types of photoreceptors can be found in the retina: rods and cones. Rods provide information about the size and shape of objects, and cones pick up red, green or blue light and provide information about color and fine detail.
Humans have binocular vision because of the location of their eyes. Each eye picks up the same image slightly out of phase with the other. Each eye has an optic nerve that crosses over to the opposite side of the brain and connects to the occipital lobe.
When the image travels to the brain, the occipital lobe forms a three-dimensional version of the image. Depth perception suffers in the absence of one eye. The visual system relies on the lens to have correct focus. Nearsightedness or farsightedness occurs otherwise.
Other issues with the visual system include cataracts or astigmatism. Cataracts are thin films that form over the cornea and stop light from entering the eye. Astigmatism is a condition in which the cornea is curved too much to refract light onto the retina. Eyeglasses and contact lenses alter the effective shape of the eye, allowing light to focus properly onto the retina.