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.
Light Speed And Light Years
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.
Moving Slower
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.
Air, Glass, And A Diamond
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.
Noticing The Speed
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.