Rods and cones in retina: colorized scanning electron micrograph (SEM) of rods and cones, showing the structure of the eye retina. Rods (green) are long nerve cells which respond to dim light, enabling images to be detected. Cones (blue) are shorter cone-like cells which detect colour. Rods and cones pass visual signals through the optic nerve to the brain. Pigment cells block light from passing further. Magnification: unknown.