The human eye is sensitive to visible light and so one can see all the external objects in the day. This is due to the function of cells in the retina. The retina has two types of cell with it, they are rods and cones, the rods are cells responsible for vision in dim light and the cones are devices responsible in bright light.
Hence, night vision is completely contributed by the rods in the eye. The human eye cannot see the surrounding region in complete darkness but, in very low light or in complete dimness they can see the surroundings in black or in greenish black color. This is because cones of the human eye are commonly concentrated in green than in any other color.