The eyeball has three layers, each formed by various components. The outer layer consists of the sclera, which is the white of the eye, the cornea, which is the clear domelike structure over the front of the eye, and the iris.
The iris is the colored part of the eye that regulates how much light enters by changing its size. Further in lies the crystalline lens of the eye, which helps focus the image onto the retina.
The retina is part of the inner layer, and converts the light that is focused from the lens into electrical impulses that the optic nerve transmits to the brain.
The optic nerve is also part of the inner layer. The middle layer is the uvea, which contains the blood vessels that provide nourishment to the eye.
The eye works and has parts much like that of a digital camera. The iris functions like a diaphragm and lets in a certain amount of light based on the surrounding conditions. In order to collect the right amount of light to produce the best possible image, the iris will open and close based on the ambient light. If there is a large amount of light, or bright lights shining towards the eye, the iris will become smaller (constrict). If there is very little light available, the iris will become wider (dilate) to gather as much light as possible.
The image (light) then passes through the lens, which focuses the image on the retina; located at the back of the eye.
Photoreceptive cells on the retina then convert the image into electrical impulses that are transmitted to the brain to decode. The photoreceptive cells, made up of cones and rods, are the primary vision cells that collect and process the light.
Cones are at their highest concentration at the fovea, which is the central zone of the retina, where we have the most visual acuity. Cones are less sensitive to light than rods, which allow them to function best in bright light conditions. Additionally, cones are responsible for transmitting color vision and spatial resolution.
This is why the eye continually regulates the iris and the lens, to regulate the amount of light and lengthen or shorten the lens to focus the image. In contrast, the rods work best in low light situations.
Rods require only one one-thousandth the amount of light that cones do to be stimulated. They are located on the retina’s periphery and do not distinguish color. Rods are able to discern form and movement, but not fine details. The fovea does not contain any rods; as a result, it is, in essence blind at night. Also, image perception disappears completely if the eye remains still form more than a few seconds. For this reason, the brain and eye work together using a technique similar to scanning, in which the eye makes very slight movements to keep the point of focus just outside of the central part of the retina.