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We all know that light bends when it travels through glass, water, or other transparentmaterial - that's how a microscope, lighthouse and spectacles work!
我们都知道光在穿过玻璃、水和其他透明材料时会发生折射——这是显微镜、灯塔和眼镜的工作原理！
And you might even know that light bends because it travels slower through glass or water than through air - but why does light slow down?
你可能还知道光的折射是因为玻璃或水中的光速比真空中要慢——但为什么光速会减慢？
And how does it speed up again when it comes out on the other side? There's nothing there to give it a push!
当光从另一边穿出来时，它如何再次加速？没有东西给光加速！
Well, if you think light is a wave, it's easy to explain - electromagnetic waves simply travel slower through glass than through air.
如果你认为光是波，那么就很容易解释了——电磁波穿过玻璃的速度比穿过空气的速度要慢。
So the wave crests are closer to each other, but the light still oscillates the same number of times per second - it stays the same color.
所以波峰距离彼此很近，但是光每秒振荡次数仍然不变——其颜色保持不变。
When the wave hits the air again, its color still doesn't change, while the crests spread out and it returns to light speed.
当波再次通过空气中，其颜色仍然保持不变，但波峰会舒展开，光恢复到原来的速度。
The simplified explanation is that the energy of a wave is determined by its frequency, or color, which doesn't change!
简单的解释是，波的能量是由它的频率或颜色决定的，而频率或颜色是不变的！
So it doesn't need a boost to speed up at the other side. But wait, you say!
所以它不需要额外的能量加速。但是等等，
I thought light traveled at the same speed in every reference frame - you still haven't explained how it can slow down!
光速在任何参考系下不变——你还没有解释光是如何减速的！
Well, now let's think about light as a particle.
现在，让我们把光当做是一个颗粒。
When light goes through glass, it gets knocked around and bumps into all sorts of molecules and electrons.
但光穿过玻璃，它会撞上分子、电子等各种粒子。
So whenever it's traveling, it's traveling at the speed of light - but it's busy interacting with and scattering off lots of stuff along the way,
所以不论何时，它都以光速移动——但它忙于与沿途的很多东西相互作用和散射，
and it doesn't necessarily take the shortest path through the glass.
并且它走的并不一定是最短路径。
It's like the President of the United States trying to cross a room – if the room is empty, he can walk across directly.
就像是美国总统要通过一个房间——如果这个房间时空的，那么他可以直接通过。
But if the room is full of people all of whom want to shake the President's hand,
但如果这间房里站满了人，他们都想和总统握手，
even though he walks from person to person at full 'Presidential-speed', he'll get slowed down along the way.
即便他在人群中以常速通行，他一路的速度仍会减慢。
As soon as he reaches the far side of the room, though, he's free to resume his pace. Full speed ahead, Mr. President!
一旦他到达房间的另一端，他就可以恢复自己的速度了。全速前进，总统先生！
