The retina is at the posterior part of the eye. It contains cells that respond to light. It contains two types of photoreceptors, which are rods and cones. Rods are most sensitive to light; a human eye contains about 125 million rods; they are necessary for seeing in dim light.
Cones are best in bright light and are not sensitive to light. However, they are sensitive to some types of colors, which are green, red, or blue. There are between 6 to 7 million cones in the eye and are used mainly for receiving sharp, accurate images and for distinguishing colors. The brains receive signals from the cones, which then translate these messages into the perception of color; this is why colors cannot be seen well in dark places.