Why does it take the eyes several minutes to get used to darkness?
[ 2006-08-23 11:38 ]
從光亮的地方進到黑暗的地方，我們的眼睛要適應幾十秒才能夠看清楚。在這幾十秒中，我們眼睛到底做了怎樣的調整讓我們適應黑暗呢？
 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. 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 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 - 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.
The retinal used in the eye is derived from vitamin A. If a person's diet is
low in vitamin A, there is not enough retinal in the rods and therefore not
enough rhodopsin. People who lack vitamin A often suffer from night blindness -- they cannot see in the dark.
rod：視網膜桿
cone：視網膜錐
retina：視網膜
rhodopsin：視網膜紫質
opsin：視蛋白
night blindness：夜盲癥
（英語點津 Annabel 編輯）
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