Martin - Light travels at different speeds depending on what itís traveling through - for example, it travels at different speeds in air than in water. The fastest that light can travel is in a vacuum. (A vacuum is what you get when you suck all of the air and water and everything else out of something - for example, space is pretty much a vacuum.) In a vacuum, light travels at 300 million meters per second, or 670.76 million miles per hour. In air, itís just a little bit slower, 670.57 million miles per hour. Hereís the speed of light in some other things youíre probably familiar with:
water 503.20 million miles per hour
ethanol 493.21 million miles per hour
*glass 441.29 million miles per hour
table salt 438.41 million miles per hour
quartz 434.43 million miles per hour
diamonds 277.17 million miles per hour
*This is for crown glass B K 7.
Different types of glass will change the speed of light somewhat differently. This is one of the more common types of glass. Also, just recently, there was a group of scientists at the Rowland Institute for Science who were able to slow light down to only 38 mph! They did this by cooling sodium atoms (sodium is the stuff that salt is made of) down really really really cold. Then they shone laser lights across the sodium and another, seperate light through that. Of course, this isnít the sort of thing that you could try out at home, but if youíre interested in learning more about it, you can check out this paper
on their research.
(republished on 07/29/06)