Rod cells. Coloured scanning electron micrograph SEM of rod cells pink in the retina of the eye. Rod cells are light_sensitive cells which respond to dim light, and so 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. Colours are detected by cone cells not seen. 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.