Rod cells. Coloured scanning electron micrograph (SEM) of a group of rod cells in the retina of the eye. Rod cells are light sensitive cells which respond to dim light, and are mainly used for dark adapted vision. They cannot detect colours, which is why vision in low light conditions is limited to shades of grey. When light falls on the tip of a rod cell, a chemical reaction sends an electrical impulse to the optic nerve (not shown), which relays it to the visual cortex of the brain. Magnification unknown.