Slow Light
About Light Speed
Light travels fast. In a vacuum it moves about 3 x 108 meters (186,000 miles) each second, a distance so large it’s difficult to comprehend. Here are some roughly equivalent distances:
- Eight times around Earth’s equator
- Most of the way from Earth to the moon
Light takes less than a millionth of a second to go from the flashlight to the mirror and back. Could you observe this time?
Total distance the light travels =
2 x 100 m = 200 m
Speed of light = c = 3 x 108m/s
Time = distance divided by speed = 200 m/(3 x 108m/s)= .7 x 10-6s
So you can expect the reflection to return about one millionth of a second after you turn on the light. Not much time to do your experiment! That’s what Galileo and his assistant found, at the turn of the seventeenth century, when they tried this same experiment from one hilltop to another.
Slow Light
Light moves with speed c in a vacuum. But light can move with a speed less than c, when it passes through some material. The table shows the speed of light when it goes through glass, water, and air. Note that in this table the speed of light in each material is given as a decimal times the speed of light in a vacuum.
In certain exotic materials light can move much slower than c, and in some cases can even be brought entirely to a halt. Light has been slowed to one mile per hour (.0000000015c) in an unusual form of matter known as a Bose Einstein Condensate (BEC). And in 2001, physicists for the first time managed to stop light in a vapor of rubidium gas.
