英语听力文摘 English Digest 192、光速与洲际高速

（单词翻译）

　　世界上跑得最快的是什么？答案是光速。它每秒能跑186,280英里。因为即使在太空中，光速也大致相同，所以光年也就用于描述不同星球之间的距离。记住了哦，光年是距离单位，不是时间单位。同时光这么快的速度又对我们有什么意义呢？一起听听吧。
If you’re driving on an interstate highway, the only thing keeping you from going faster than the speed limit is your sense of civic¹ duty, or perhaps that police car up ahead.
There’s one speed limit, however, that most scientists agree nothing will ever break.  That’s the speed of light:186,280 miles per second.

　　As it travels through deep space, light moves at exactly this speed, no more and no less.The speed of light is so reliable, we even use it to measure distances between the stars.
If it takes starlight fifty years to reach us, we say that this star is fifty light years away.
You might be surprised to learn, however, that light itself does not always travel at the speed of light.Our interstate highways generally have a higher speed limit, but once you enter a city or residential² street, the speed limit lowers.
Likewise³, light travels at its maximum speed through a vacuum⁴ the interstellar highway but it slows down when it enters air, water, or some other medium.
Through air, light moves a hint⁵ slower:  forty miles per second less than its maximum speed.Through glass, light moves almost eighty thousand miles per second slower.
Light moves more than a hundred thousand miles per second slower through a diamond less than half its speed in a vacuum.
Light moves so quickly anyway, do we ever notice this change in speed? Actually, we do.When light slows down it also bends slightly.This is know as refraction.
Lenses⁶ use the lower speed limit of light in glass to bend and focus the light.If light always moved at it’s maximum speed, none of our telescopes, eyeglasses or even our eyes themselves would ever work.