We can see in very bright sunlight, and we can also see in nearly total darkness. If you spend much time working with a camera you know how amazing this range is. Film that works well outdoors is nearly useless indoors, and vice versa. The pupil contracts and expands depending on the amount of light, and can physically block the amount of light entering the eye in bright situations. The rod and cone cells in the retina.
Cone cells can perceive color in bright light.
Rod cells perceive black and white images and work best in low light. A chemical called rhodopsin in the rods. Rhodopsin is the key to night vision - it is the chemical that the rods use to absorb photons and perceive light. When a molecule of rhodopsin absorbs a photon, it splits into a retinal and an opsin molecule. These molecules later recombine naturally back into rhodopsin at a fixed rate, and recombination is fairly slow.