One of the most amazing things about human vision is the incredible range it has. We can see in very bright sunlight, and we can also see in nearly total darkness. The range that our eyes have comes from three different parts of the eye.Pupil. 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.Rod and cone cells in the retina.Our eyes sense light with two different types of cells: rods and cones. Cone cells can perceive color in bright light. Rod cells perceive black and white images and work best in low light. Rhodopsin is a chemical found 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. So, when you expose your eyes to bright light, all of the rhodopsin breaks down into retinal and opsin. If you then turn out the lights and try to see in the dark, you can't. The cones need a lot of light, so they are useless, and there is no rhodopsin now so the rods are useless, too. Over the course of several minutes, however, the retinal and opsin recombine back into rhodopsin, and you can see again.
One of the most important points with eyes getting used to the dark is actually how your eyes see in the first place. Which is when that you're looking at something, There are beams or rays of light of certain wavelengths or colours coming into your eye and they interact with the photo pigment. A chemical which is sensitive to certain wavelengths which is in your retina. When the light waves hit that pigment, they cause the pigment to change its configuration. It is so called bleach. When it changes its configuration, it then signals the cell to change its behaviour so that's basically how the retina turns light waves into brain waves. It's turning the information into pulses of nerve activity the brain can understand. For a period of time, when that pigment has been bleached, it can't to respond to light again until its regenerated, until its shape goes back to its original starting confirmation. So, when you go from a very light area where on average many of your pigment molecules in your retina will be being bleached out any given time and then you go into the dark; Many of those bleached-pigment molecules will slowly turn back into unbleached pigment molecules, they're sensitive again. So, in other words, the longer you spend in the dark, the more pigment molecules becomes sensitive and therefore, the more sensitive your eyes become.
At the back of the eye on the retina, there are two types of photoreceptors (cells which detect light). Cones deal with colour and fine detail and act in bright light, while rods deal with vision in low-light situations. In the first few minutes of moving into a dark room, cones are responsible for vision but provide a poor picture. Once the rods become more active (they are initially slow to react), they take over and create a much better picture in poor light. Once you move back into light, the rods are reset and so dark-adaption will take a few moments again.