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The Blurry World Underwater
水下的朦胧世界
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?
当你在水底睁开眼睛的时候，所有的东西看上去都很模糊，但是戴上护目镜，所有的东西看起来都变得很清晰。为什么我们水手在水底不戴护目镜就不能自动聚焦呢？
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，我们需要探索当我们处于高处和干燥的地方时，我们的眼睛是怎样聚焦的。我们聚焦的能力取决于光线通过不同密度的物质时发生的折射或者弯曲。观察到折 射现象的一种方法是在装水的玻璃杯中放入一个汤匙，汤匙在水面处看上去似乎是被折成了两段。但事实上，这是光线在通过空气和密度更高的水时发生的折射现 象。透镜以同样的方式运作。但不同的是，不像水平面使所有的光线以同样的角度折射弯曲，透镜是曲面的，这样就使得光线从不同的角度发生了弯曲，所以它们就 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.
在眼睛的前部有两个透镜，通过外部的角膜和另一个位于内侧的晶状体，我们得以把图像聚焦到位于眼睛后部的视网膜上。当光通过空气进入角膜时，角膜密度比空 气大，大多数折射就发生在这个时候。但是水的密度和角膜的相似，所以，当我们在水底睁开眼睛的时候，入射光线很难弯曲或者聚焦。内部的晶状体可以使光线发 生一些弯曲，但这不能弥补在角膜处失去的折射，所以到达视网膜的光无法聚焦，水底世界看起来就很模糊。
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.
护目镜可以保留在角膜前方的空气，恢复了不同浓度物质的界面，实现了光的折射，从而使我们看到的东西变得清晰。
