Welcome to brain Stuff from how Stuff works dot com where smart Happens. Hi Marshall Brain. 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. But you might have also noticed that your eyes can take several minutes to adapt to the dark. For example, if you walk from a bright room into a dark room, you can't see anything for a minute or two. Why does that happen.
The range that our eyes have comes from three different parts of the eye. First, there's the 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. In dark situations, your pupils open up to let in more light. Then there are the rod and cone cells in your retina. Our eyes sense light with two different types of cells, rods and cones. Cone
cells perceive color in bright light. Rods cells perceive black and white images and work best in low light. Finally, there's a chemical called rhodopsin The chemical found in the rods that lets them detect light. Rhodopson is the key to night vision. It is the chemical that the rods used to absorb photons and perceive light. When a molecules option absorbs a photon, it splits into a retinal molecule and an option molecule. These molecules later recombined naturally back
into rhodopsin at a fixed rate. But the recombination process is fairly slow, so when you expose your eyes to bright light, all the rehodopsin breaks down into retinal and opstion. If you then turn out the lights and try to see in the dark, you can't. The cones need a lot of lights, so they're useless, and there's no rhodopsin now, so the rods are useless too. Over the course of several minutes, however, the retinal and the opson molecules recombined
back into rhodobson, and you can see again. Here's a fun fact. The retinal used in the eye is derived from vitamin A. If a person's diet is low in vitamin A, there's not enough retinal in the rods and therefore not enough for dobson. The people who lack vitamin A often suffer from night blindness. They cannot see in the dark at all. Do you have any ideas or suggestions for this podcast? If so, please send me an
email at podcast at how stuff works dot com. For more on this and thousands of other topics, go to how stuff works dot com