The retina, a 5 square centimeter area in the back of the eye, is where all light detection takes place. The retina is a network of nerves connected to over 100 million photo-sensitive rods and cones. The signals created by these rods and cones are then sent via the optic nerve to the brain.
The top layer of the retina surprisingly does not interpret the light that strikes it. This layer, called the Plexiform layer, is a web of optical nerves. These nerves carry the signals of rods and cones to the optic nerve. This web is located between the photo-sensitive cells and the vitreous humor so that the web's cells can be nourished. Fortunately, these cells are nearly transparent so only minimally interfere with light striking the photo-sensitive cells.
The bottom layer of the retina is called the choroid. The choroid serves a double purpose: nourishment and absorption. The choroid carries blood to the retina and the humors, providing nourishment to the eye. In addition, the choroid absorbs any light that strikes it. This is extremely important, because light that passes through the rods and cones does not reflect back. If the light did reflect, the photo-sensitive cells would receive the light message twice, and would think that there was twice as much light as there really was.
Rods and Cones
The final layer of the retina is the photo-sensitive layer, made up of rods and cones. When they are exposed to light, rods and cones send signals to the plexiform layer. This signal is then transferred via the optic nerve to the brain to be interpreted.
Cones are the cells that we use to distinguish color. This has several very interesting effects. Almost all cones are located at the middle of the retina, so it is very difficult to distinguish color in our peripheral vision. In addition, it makes it impossible to see color in low levels of light when the cones are "turned off."
The Blind Spot
The Optic Nerve receives all of the signals created in the eye and transfers them to the brain to be processed. The nerve enters in the back of the eye, and where it enters there are neither rods nor cones. This results in a "blind spot," an area where our eyes cannot see. Though this area is rarely noticed because the brain "fills in" the blind spot, an interesting trick can be done to demonstrate its existence.