Retinal photoreceptor

At the back of the eye, in the retina, there are cells that do something that sounds impossible: turn light into a nerve signal. They are the photoreceptors, and they come in two shapes that give them their names: cones and rods. Typical mammalian retinas have two or three types of cone and a single type of rod.

Each of those cells has two halves with different jobs. In the outer segment, or tip, light is absorbed and converted into nerve signals. The inner segment holds the mitochondria, the cell's power plants, which fuel that process, called phototransduction. And the split between cones and rods comes down to a day shift and a night shift. Rods gather photons when light is scarce: they are more numerous and more sensitive than cones, but slower. Cones work best in bright daylight and, because different types respond to different parts of the spectrum, they are the ones that tell colors apart.