We humans have two eyes and not one or three, like much of the hunting animal world because it is the minimum required to have depth perception – the ability to gauge the distance between yourself and your prey. Vision involves the nearly simultaneous interaction of the two eyes and the brain through a network of neurons, receptors, and other specialized cells.
The first steps in this sensory process are the stimulation of light receptors in the eyes, conversion of the light stimuli or images into signals, and transmission of electrical signals containing the vision information from each eye to the brain through the optic nerves.
The human eye is equipped with a variety of optical components including the cornea, iris, pupil, aqueous and vitreous humors, a variable-focus lens, and the retina. Together, these elements work to form images of the objects that fall into the field of view for each eye.
When an object is observed, it is first focused through the convex cornea and lens elements, forming an inverted image on the surface of the retina, a multi-layered membrane that contains millions of light-sensitive cells.
In order to reach the retina, light rays focused by the cornea must successively traverse the aqueous humor (in the anterior chamber), the crystalline lens, the gelatinous vitreous body, and the vascular and neuronal layers of the retina before they reach the photosensitive outer segments of the cone and rod cells. These photo sensory cells detect the image and translate it into a series of electrical signals for transmission to the brain. Ironically, despite the fantastic, seemingly evolution-defying complexity of the human eye, it is merely a light collection device. It is the brain that actually “sees” i.e. takes the light and converts it into intelligible information upon which the rest of the brain can act.
The eye being just a collection device and light being just another electromagnetic wave opens up the possibilities of science fiction becoming science fact: soon humans will have the ability to equip ourselves with “Geordie” style visors and ocular implants that enable us to see all of the electromagnetic wave spectrum. In fact scientists are now successfully experimenting with getting mice to “see” infrared light through direct cortical stimulation. See the full Scientific American article here.
Color blindness, a disruption in the normal functioning of human photopic vision, can be caused by host of conditions, including those derived from genetics, biochemistry, physical damage, and diseases. Partial color blindness, a condition where the individual has difficulty discriminating between specific colors, is far more common than total color blindness where only shades of gray are recognized.