Take a look out your window, put on your glasses if you wear them.
向窗外张望一下，有必要的话，戴上眼镜。
You might want to grab a pair of binoculars, too, or a magnifying lens.
也许你还想拿副望远镜或是放大镜。
Now, what do you see?
现在，你看到了什么
Well, whatever it is, it's not the multiple layers of glass right in front of you.
无论你看到了什么答案，都可能不是那一层层近在咫尺的玻璃。
But have you ever wondered how something so solid can be so invisible?
但是你是否想知道，为什么有如此固态的东西却看不见呢？
To understand that, we have to understand what glass actually is, and where it comes from.
要明白这点，我们首先要知道玻璃究竟是什么，以及它是怎样形成的。
It all begins in the Earth's crust, where the two most common elements are silicon and oxygen.
所有一切都始于地壳在它其中最基本的两种元素：硅和氧。
These react together to form silicon dioxide, whose molecules arrange themselves into a regular crystalline form known as quartz.
它们相互作用形成二氧化硅，二氧化物的分子自动排列形成晶体状物，被称为石英。
Quartz is commonly found in sand, where it often makes up most of the grains and is the main ingredient in most type of glass.
石英通常在沙中被发现，沙大多由颗粒组成，而颗粒是多数玻璃的主要原料。
Of course, you probably noticed that glass isn't made of multiple tiny bits of quartz, and for good reason
当然，你可能注意到玻璃并不是由许多石英粒组成的，这点显而易见。
For one thing, the edges of the rigidly formed grains and smaller defects within the crystal structure reflect and disperse light that hits them
一方面，晶体结构内部严格形成的颗粒边缘或者很小的瑕疵反射及散射照向它们的光。
But when the quartz is heated high enough the extra energy makes the molecules vibrate until they break the bonds holding them together and become a flowing liquid, the same way that ice melts into water.
但是当石英加热到足够热时，产生的热量就会让分子产生震动直到它们打破以前的链结，进而形成流动的液体，就和冰融化成水一样。
Unlike water, though, liquid silicon dioxide does not reform into a crystal solid when it cools
但是和水不同的是，液态的二氧化硅冷却之后不会形成固态晶体。
Instead, as the molecules lose energy, they are less and less able to move into an ordered position, and the result is what is called an amorphous solid.
相反，当分子失去能量后，它们就更不太可能移动形成一个有秩序的状态，这样的结果就是人们所说的非晶型固体。
A solid material with the chaotic structure of a liquid, which allows the molecules to freely fill in any gaps.
一种具有混乱液体结构的固态物质使得分子可以自由填补任何缝隙。
This makes the surface of glass uniform on a microscopic level, allowing light to strike it without being scattered in different directions.
这样就让玻璃表面在微观层面上是一致的，光可以穿透它但不会散射出去。
But this still doesn't explain why light is able to pass through glass rather than being absorbed as with most solids.
但是这还是不能解释为什么光可以穿透玻璃而不是像其他大多固体那样被吸收。
For that, we need to go all the way down to the subatomic level.
关于这个，我们需要进一步谈论到亚原子层面上。
You may know that an atom consists of a nucleus with electrons orbiting around it, but you may be surprised to know that it's mostly empty space.
你或许知道原子是由原子核以及围绕着它的电子共同形成的，但是你有可能很意外的发现它的大部分体积都是空的。
In fact, if an atom were the size of a sports stadium, the nucleus would be like a single pea in the center, while the electrons would be like grains of sand in the outer seats.
事实上，如果把一个原子比作一座体育场那么大，那么原子核可能就是体育场中心的一颗豆子，而中子就像是沙粒环绕于四周的座位上。
That should leave plenty of space for light to pass through without hitting any of these particles.
这样一来，就会留有大量的缝隙让光可以穿过它却不会碰到其他颗粒。
So the real question is not why is glass transparent, but why aren't all materials transparent?
所以真正的问题并不是为什么玻璃是透明的，而是为什么所有物质都不透明？
The answer has to do with the different energy levels that electrons in an atom can have.
答案可以归结于与原子内电子所拥有的不同能级有关。
Think of these as different rows of seats in the stadium stands.
若把它们视为体育场里一排排的座位。
An electron is initially assigned to sit in a certain row, but it could jump to a better row, if it only had the energy.
一个电子首先被安排到一个固定的座位，但是它可以跳到另一个更好的座位，只要它有足够的能量。
As luck would have it, absorbing one of those light photons passing through the atom can provide just the energy the electron needs.
如果幸运吸收一个穿过这个原子的光子刚好可以提供电子所需要的能量。
But there's a catch.
但有个问题。
The energy from the photon has to be the right amount to get an electron to the next row.
来自光子的能量必须正好合适才能使得电子跳到下一排。
Otherwise, it will just let the photon pass by, and it just so happens that in glass, the rows are so far apart that a photon of visible light can't provide enough energy for an electron to jump between them.
否则，光子就会穿过这些电子就像光穿过玻璃一样排与排分开的太远，以至于可见光的光子不能提供足够能量让电子在两排之间跳动。
Photons from ultraviolet light, on the other hand, give just the right amount of energy, and are absorbed, which is why you can't get a suntan through glass.
但另一方面，紫外线光子刚好可以提供足够能量进而被吸收，这就是为什么透过玻璃你不会被晒黑的原因。
This amazing property of being both solid and transparent has given glass many uses throughout the centuries.
这种既是固体又是透明状的特性让玻璃在几个世纪以来得到了大量的应用。
From windows that let in light while keeping out the elements, to lenses that allow us to see both the vast worlds beyond our planet, and the tiny ones right around us.
从可以让光照入却又能把其他物质挡在外面的窗户，到能让我们同时看到地球以外的广袤星空，以及眼前的微小世界的镜片。
It is hard to imagine modern civilization without glass.
很难想象没有玻璃的现代文明会是怎样。
And yet for such an important material we rarely think about glass and its impact.
但是对于这样如此重要的物质，我们却很少思考它以及它的作用。
It is precisely because the most important and useful quality of glass is being featureless and invisible that we often forget that it's even there.
这恰恰是因为玻璃最重要的以及最有用的特质，是使其变得无特征并不可见，以至于我们都忘了它的存在。