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Why does cloth appear darker when wet?
[ 2006-07-31 11:14 ]
濕衣服的顏色看起來要比干衣服深一些，不知道大家在洗衣服的時候有沒有注意過這個。這是什么原因呢？難道真是傳說中的水給衣服染了色？當然不是。其實，這是光的惡作劇。
 When light illuminates dry cloth, a typical light beam will hit a fiber,refractinto it, and then refract back out of it. Because cloth has a highindexof refraction, the light usually gets bent a lot at both refractions, so it comes out in a different direction than it went in. Also, because cloth is not perfectly transparent, the light comes out a little dimmer than it went in. A light beam will undergo this process many times, passing through many fibers in succession, and so effectively, a lot of the light is scattered off the cloth in a random direction. Some of this light hits your eye, and then you can see the cloth.
When you get the cloth wet, the air between the fibers is replaced by water. Water has a much higher index of refraction than air, and so the relative index of refraction between water and the fiber is much lower than between air and the fiber. Thus, each time when a light beam enters a fiber, it is bent much less than it otherwise would be. Therefore, the light beam has to pass through many more fibers before it is turned all the way around, and can come back out of the cloth to hit our eyes. Remember that each time the light beam passes through a fiber, it gets dimmer. Since the light is now passing through a lot more fibers, it gets a lot dimmer before it leaves the cloth, so a lot less light is coming back to our eyes. Hence, the cloth appears darker.
refract:折射
index:指數
scatter:散射
（英語點津 Annabel 編輯）
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