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Speaker 1: Welcome to brain Stuff from how Stuff works dot com

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Speaker 1: where smart Happens. Hi Marshall Brain. One of the most

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Speaker 1: amazing things about human vision is the incredible range it has.

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Speaker 1: We can see in very bright sunlight, and we can

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Speaker 1: also see in nearly total darkness. But you might have

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Speaker 1: also noticed that your eyes can take several minutes to

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Speaker 1: adapt to the dark. For example, if you walk from

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Speaker 1: a bright room into a dark room, you can't see

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Speaker 1: anything for a minute or two. Why does that happen.

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Speaker 1: The range that our eyes have comes from three different

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Speaker 1: parts of the eye. First, there's the pupil. The pupil

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Speaker 1: contracts and expands depending on the amount of light, and

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Speaker 1: can physically block the amount of light entering the eye

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Speaker 1: in bright situations. In dark situations, your pupils open up

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Speaker 1: to let in more light. Then there are the rod

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Speaker 1: and cone cells in your retina. Our eyes sense light

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Speaker 1: with two different types of cells, rods and cones. Cone

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Speaker 1: cells perceive color in bright light. Rods cells perceive black

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Speaker 1: and white images and work best in low light. Finally,

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Speaker 1: there's a chemical called rhodopsin The chemical found in the

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Speaker 1: rods that lets them detect light. Rhodopson is the key

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Speaker 1: to night vision. It is the chemical that the rods

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Speaker 1: used to absorb photons and perceive light. When a molecules

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Speaker 1: option absorbs a photon, it splits into a retinal molecule

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Speaker 1: and an option molecule. These molecules later recombined naturally back

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Speaker 1: into rhodopsin at a fixed rate. But the recombination process

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Speaker 1: is fairly slow, so when you expose your eyes to

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Speaker 1: bright light, all the rehodopsin breaks down into retinal and opstion.

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Speaker 1: If you then turn out the lights and try to

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Speaker 1: see in the dark, you can't. The cones need a

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Speaker 1: lot of lights, so they're useless, and there's no rhodopsin now,

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Speaker 1: so the rods are useless too. Over the course of

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Speaker 1: several minutes, however, the retinal and the opson molecules recombined

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Speaker 1: back into rhodobson, and you can see again. Here's a

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Speaker 1: fun fact. The retinal used in the eye is derived

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Speaker 1: from vitamin A. If a person's diet is low in

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Speaker 1: vitamin A, there's not enough retinal in the rods and

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Speaker 1: therefore not enough for dobson. The people who lack vitamin

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Speaker 1: A often suffer from night blindness. They cannot see in

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Speaker 1: the dark at all. Do you have any ideas or

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Speaker 1: suggestions for this podcast? If so, please send me an

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Speaker 1: email at podcast at how stuff works dot com. For

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Speaker 1: more on this and thousands of other topics, go to

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Speaker 1: how stuff works dot com