When light rays have passed through the cornea and lens in the eye, they shine onto the rear inner surface of the eyeball, a layer called the retina. It is not much bigger than a postage stamp, and ever thinner. Yet it contains more than 130 million microscopic cells. When light shines on them, they generate nerve signals – that is, they are light sensitive.
There are two types of light sensitive cells in the retina, named after their shapes – rods and cones. There are about 125 million rods and they all respond to all types of light, regardless of whether it is white, red, blue, green, or yellow. Rods work in very weak light, so they help the eye to see in dim conditions. The other type of light – sensitive cell is the cone. There are about 7 million of them in the retina, clustered mainly around the back, opposite the lens.