In the eye , light rays are concentrated in the retina with a lens like a camera. This is where the realization of the scene begins. The layer of light-collecting cells arranged in the retina converts light-particles into electrochemical signals. These cells are of two types. Namely: 'Rod' and 'No'. The 'no' cells are useful for seeing in the dark. They are located along the edges of the retina screen. Some of these cells can also be seen in low light.