What we observe as a result of all of this is that, whenever light passes through a transparent medium, it slows down.

我们观察到的结果是，当光通过透明介质时，它会变慢。

But that’s not actually true: light travels slower when it’s passing through a transparent medium, like air, or glass, or water.

但事实并非如此： 光在穿过透明介质（如空气、 玻璃或水） 时传播速度较慢。

The key to understanding what that means lies with the term refraction, the ability of a transparent medium, like glass, water, or the eye to change the direction of light passing through it.

理解这代表什么的关键取决于这个术语折光能力，一个透明媒体的能力，如玻璃，水，或者眼睛，当光穿过它时，来改变光的方向。