Eye rods are cells that help the eye process light. These types of cells are also called photoreceptors. These are located in the retina, which is the lining on the inside of the eye. Each human eye contains about 120 million rods. Another type of photoreceptor, eye cones, also process light. There are about 7 million eye cones in a single human eye.
Extremely sensitive to light, eye rods are responsible for allowing people to see in low-light conditions. Rods only see in black, white, and gray, so they do not help humans distinguish colors. This is why humans are not able to distinguish colors, apart from shades of gray, when there is no light.
The rods do not adapt as quickly to changing light conditions as cones do. It can take 30 minutes or longer for rods to adjust to changes in light before optimum low-light or dark vision is achieved. Cones adapt faster, but do not do so instantly. This is why things appear blurry and overly bright when moving from a dark environment to a well-lit one, after spending several minutes or more in the dark.
Cones are less responsive to light than eye rods are, but they are responsible for allowing humans to distinguish colors. There are three types of cones — one each for blue, red, and yellow. These different types of cones work together to allow people to see and recognize other colors, since all colors are some combination of the three primary ones.
Distinguishing an object’s shape and sensing movement would not be possible without eye rods. Rods are also used more in peripheral vision than cones are. Since cones allow for better visual resolution, humans subconsciously adjust their eyes to utilize the cone cells when focusing on an object. This is why a person may be able to see a dim object when using his or her peripheral vision, but may not be able to see it when looking at the object directly.
A small part of the retina does not have any rod or cone cells. This portion accounts for person's blind spot. If someone closes one eye and focuses on one object, with another item a few inches away, the object that is not being focused on will disappear at some point when the person moves his or her face closer to them. The zone in which the second object disappears is in blind spot, and is not seen.