Vision, or sight, is the process by which a person “sees” using their eyes. Sight works much like a camera – vision is achieved using a refractory lens that focuses light waves (the picture) onto nerve endings (the film), which communicate with the brain for interpretation. Sight is the basic function of the eye, and the refractory system is inside the eyeball (image available here). The nerve endings are located in the back of the eye, on the retina.
The Refractory System
The refractory system of the eye consists of specialized structures and fluid, and sits between the cornea and retina. The cornea is a thin, clear sheath on the outer eye that lacks blood vessels. This lack of interference allows light to pass through the cornea unhindered. Behind the cornea is the pupil, seen as the black portion of the eye, which is actually an opening. The size of the pupil is controlled by two sets of muscles, the iris, which is the pigmented portion of the eye. Also behind the cornea, and through the pupil, is a cavity containing the fluid that keeps the cornea nourished – the aqueous humor.
The other side of the cavity is where the lens is found. The lens is what refracts the light entering the eye, aiming it to hit the retina. The lens contains no blood vessels and is also nourished by the aqueous humor. The lens is adjusted by the ciliary body, muscles that allow light to be focused on the retina as involuntarily determined for distance and movement. When the lens is misshapen or the wrong thickness, vision disorders result, including near-sightedness and far-sightedness. Behind the lens is a semi-solid chamber called the vitreous humor. This substance puts a pressure on the retina to hold it in place, allowing stability for the photoreceptors.
Photoreceptors
The back of the inside of the eyeball, the retina, is lined with photoreceptors, nerve ganglia that respond to light. The receptors are found as two types – rods and cones. The rods detect light intensity, regardless of wavelength, and are responsible for determining whether the light in the external environment is bright or dim. The cones are only active in bright light and respond to particular wavelengths of visible light. Three particular wavelengths are detected by cones: red, blue, or green. Each type of cone detects only one particular wavelength of light. When a particular type of cone is missing color blindness results, allowing only bright, dim, and the remaining wavelengths to be determined by the eye. The macula region of the retina, which is approximately 5 mm in size, has two layers of ganglion cells, making it more receptive to light. There is also a specialized pit in the macula where the greatest concentration of cones occurs. This is called the macula fovea. This specialized pit is important for high visual acuity.
Optic Nerve Signals
When stimulated with light, the retinal photoreceptors transmit signals to the vision center of the brain in the occipital lobe via the optic nerves. There is a blind spot at the back of each eye at the point where this nerve passes through the eyeball because there are no receptors present at the spot (test for the blindspot here). The blind spot is compensated for by binocular vision when both eyes are fully functioning because the image from each of the two optic nerves overlaps slightly with the other.
Vision In Action
To put it all together: light waves enter the cornea, being the specific wavelengths to indicate the color and intensity of the light reflected by the object being visualized. If it is dim, the iris opens the pupil to allow more light waves in; if it is bright, it closes the pupil to allow enough in to see but not enough to overstimulate the retina and blind the viewer. The lens is adjusted by the ciliary body to focus the wavelengths on the retina. The photoreceptors send the intensity and wavelength information to the brain, which produces the image accordingly. The object is now seen through the process of vision.
