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Q & A: Slowing the Speed of Light

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Most recent answer: 10/22/2007
How can you slow the speed of light?
- Martin (age 13)
Tølløse, Denmak, europe

Martin - Light travels at different speeds depending on what it’s 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, it’s just a little bit slower, 670.57 million miles per hour. Here’s the speed of light in some other things you’re 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 a gas of sodium atoms (sodium is one of the elements that table salt is made of) down really really really cold. Then they shone laser lights across the sodium and another, separate light through that. Of course, this isn’t the sort of thing that you could try out at home, but if you’re interested in learning more about it, you can check out this on their research.


(published on 10/22/2007)

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