英語聽力文摘 光速與洲際高速
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。因為即使在太空中，光速也大致相同，所以光年也就用于描述不同星球之間的距離。記住了哦，光年是距離單位，不是時間單位。同時光這么快的速度又對我們有什么意義呢?一起聽聽吧。
If you’re driving on an interstate highway, the only thing keeping you from going faster than the speed limit is your sense of civic 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 residential street, the speed limit lowers.
Likewise, light travels at its maximum speed through a vacuum the interstellar highway but it slows down when it enters air, water, or some other medium.
Through air, light moves a hint 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.
Lenses 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.
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