For you to see anything, your eye first forms a precise image of it on your retina (see Optical stage ). Then the light energy striking your retina is converted into nerve impulses by the retina’’s photoreceptor cells. The image is then be processed by your nervous system. This processing starts immediately in the retina, not in the brain. In fact, the retina is a part of the brain that is located outside it, somewhat the way you may regard your home satellite dish as an integral part of your television receiver.
The retina is a thin layer of nerve tissue with the consistency and thickness of a wet cigarette paper. The neurons of the retina are arranged in 3 main layers separated by 2 intermediate layers whose main purpose is to make connections among the various neurons.
The deepest layer of neurons processes the light first. These neurons are the photoreceptors, the only cells in the retina that can convert light into nerve impulses. The photoreceptor layer then transmits these impulses to the bipolar neurons in the second layer and on to the ganglion neurons in the third layer. It is only the axons of these ganglion neurons that exit the eye and carry the nerve impulses along the optic nerve to the first visual relay in the brain.
The retina contains two types of photoreceptors: rods and cones. Each type of photoreceptor is sensitive to different wavelengths of light.
Rods are cells highly sensitive to light of many wavelengths. They require only a single photon to trigger a response. Incoming light triggers the rods to send signals to the brain. Basically rods allow us to see in black, white and shades of gray. Because the rods are very sensitive they help is see in dim light. Because rods are so sensitive, they will be responding as much as they can when the light is still quite dim, so they are completely useless in full daylight.
Cones are photoreceptor cells that allow is to see colors. Cones register either blue, green or red. Your retina organizes all colors with combinations of these three basic colors. Cones are less sensitive than rods so good lighting is required for color vision.
Why do we have rods and cones?
We are diurnal animals. We operate in the daylight and sleep at night. We need receptors that are sensitive to very low amounts of light at night and to orders of magnitude more illumination in the daytime. It is the cones that are responsive to high levels of light and rods that are functioning at low levels of light.
When you enter a dark cinema from a bright day you can not see a thing at first, as there is not enough light to get your cones going and you are relying on your rods. After a few minutes you can see quite clearly. Having just been outside in bright sunlight the rods are completely bleached, and it takes some minutes in the dark for the rods to recover their sensitivity.