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We all know that light bends when it travels through glass, water, or other transparent material
众所周知，光在穿过玻璃、水或其他透明介质时会发生折射现象，
that's how a microscope, lighthouse and spectacles work!
这也是显微镜、灯塔和眼镜的工作原理！
And you might even know that light bends
你甚至可能知道折射
because it travels slower through glass or water than through air,
是因为光在玻璃或水中的传播速度比在空气中的慢，
but why does light slow down?
但光速为什么会慢下来呢？
And how does it speed up again when it comes out on the other side?
当光速通过介质后，又是如何再次加速的呢？
There's nothing there to give it a push!
那里没有什么东西可以给它加速的！
Well, if you think light is a wave, it's easy to explain,
嗯，如果你认为光是一种波，很容易解释，
electromagnetic waves simply travel slower through glass than through air,
电磁波在玻璃中的速度比在空气中慢，
so the wave crests are closer to each other,
所以彼此的波峰更接近，
but the light still oscillates the same number of times per second, it stays the same color.
但是光仍然以每秒相同的频率振荡，颜色不变。
When the wave hits the air again, its color still doesn't change,
当波再次通过空气后，波的颜色依然保持不变，
while its crests spread out and it returns to light speed.
而这时波峰舒展开，又恢复到光速。
The simplified explanation is that the energy of a wave
简单的解释是，波的能量
is determined by its frequency, or color, which doesn't change!
是由它的频率，也就是颜色决定的，不会发生改变的！
So it doesn't need a boost to speed up at the other side. But wait, you say!
所以在另一边它不需要加速。但是等等，你说！
I thought light traveled at the same speed in every reference frame,
我认为光在每个参照系中以相同的速度传播，
you still haven't explained how it can slow down!
你还没有解释它是如何减速的！
Well, now let's think about light as a particle.
好吧，现在让我们把光看成一个粒子。
When light goes through glass, it gets knocked around and bumps into all sorts of molecules and electrons.
当光穿过玻璃时，它会被撞击并碰撞到各种分子和电子。
So whenever it's traveling, it's traveling at the speed of light,
所以无论什么时候，它都以光速传播，
but it's busy interacting with and scattering off lots of stuff along the way,
但并在这个过程中会跟其他大量散布的粒子产生互动，
and it doesn't necessarily take the shortest path through the glass.
也不一定要走最短的路径穿过玻璃。
It's like the President of the United States trying to cross a room
这就像美国总统奥巴马在穿过一个房间时，
if the room is empty, he can walk across directly.
如果房间是空的，他可以直接走过去。
But if the room is full of people all of whom want to shake the President's hand,
但是，如果房间里挤满了想和总统握手的人，
even though he walks from person to person at full "Presidential-speed",
即使他在人与人之间来回走动，行进速度不变，
he'll get slowed down along the way.
他会在这个过程中慢下来的。
As soon as he reaches the far side of the room, though,
不过，他一出这个房间，
he's free to resume his pace. Full speed ahead, Mr. President!
他就可以恢复到之前的速度。全速前进，总统先生！
