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The Blurry World Underwater
水下的朦朧世界
When you open your eyes underwater, things look blurry, but with goggles, everything looks perfectly clear. Why can't we landlubbers focus underwater without goggles?
當你在水底睜開眼睛的時候，所有的東西看上去都很模糊，但是戴上護目鏡，所有的東西看起來都變得很清晰。為什么我們水手在水底不戴護目鏡就不能自動聚焦呢？
First, we need to explore how our eyes focus when we're high and dry. Our ability to focus depends on the refraction, or bending, of light rays as they pass between substances of different densities. One way to see refraction is to put a spoon in a glass of water. It looks like the spoon bends at the water's surface, but it's really the light rays bending as they pass between the air and the denser water. A lens works in a similar way, but unlike the flat surface of water, which bends all the rays the same way, a lens is curved, bending rays at different angles so they come to a focus.
首先，我們需要探索當我們處于高處和干燥的地方時，我們的眼睛是怎樣聚焦的。我們聚焦的能力取決于光線通過不同密度的物質時發生的折射或者彎曲。觀察到折 射現象的一種方法是在裝水的玻璃杯中放入一個湯匙，湯匙在水面處看上去似乎是被折成了兩段。但事實上，這是光線在通過空氣和密度更高的水時發生的折射現 象。透鏡以同樣的方式運作。但不同的是，不像水平面使所有的光線以同樣的角度折射彎曲，透鏡是曲面的，這樣就使得光線從不同的角度發生了彎曲，所以它們就 形成了聚焦。
The front of the eye has two lenses, the cornea on the outside and another lens inside, to focus images on the retina, at the back of the eye. Most of the refraction takes place when light passes from the air into the cornea, which is much more dense, but water and the cornea have similar densities. So, when we open our eyes underwater, incoming light rays are hardly bent, or focused, at all. The inside lens bends the rays a little, but it can't make up for the lost corneal refraction, so the light that reaches the retina isn't focused and the underwater world looks blurry.
在眼睛的前部有兩個透鏡，通過外部的角膜和另一個位于內側的晶狀體，我們得以把圖像聚焦到位于眼睛后部的視網膜上。當光通過空氣進入角膜時，角膜密度比空 氣大，大多數折射就發生在這個時候。但是水的密度和角膜的相似，所以，當我們在水底睜開眼睛的時候，入射光線很難彎曲或者聚焦。內部的晶狀體可以使光線發 生一些彎曲，但這不能彌補在角膜處失去的折射，所以到達視網膜的光無法聚焦，水底世界看起來就很模糊。
Goggles clear things up by inserting a pocket of air in front of the cornea, restoring that crucial interface between substances of different densities to refract the light.
護目鏡可以保留在角膜前方的空氣，恢復了不同濃度物質的界面，實現了光的折射，從而使我們看到的東西變得清晰。
