Rod cells of the eye. Coloured scanning electron micrograph of rod cells of the human eye. Rod cells are specialist light receptor cells located in the retina at the back of the eye. Each long rod-shaped cell (green) is a simple brightness detector. The tip of each cell is invested with cytoplasmic processes (orange) from the surface pigmented epithelium (not seen). When light hits a rod chemical reactions cause a message to go to the brain along the optic nerve (not seen). Rod cells are sensitive to low levels of light but they cannot detect different colours. This means that when there is very little light humans see objects in shades of grey. Magnification: unknown.