Ian
priscilla 1The Speed of Light
January 8th 2010 02:07
The speed of light (usually denoted c) is a physical constant. Its value is exactly 299,792,458 metres per second, often approximated as 300,000 kilometres per second or 186,000 miles per second. It is the speed of electromagnetic radiation (such as radio waves, visible light, or gamma rays) in vacuum, where there are no atoms, molecules or other types of matter that can slow it down.
For much of human history, it was not known whether light was transmitted instantaneously or simply very quickly. In the 17th century, Ole Romer first demonstrated that it travelled at a finite speed by studying the apparent motion of Jupiter's moon Io. By 1975, the speed of light was known to be 299,792,458 m/s with a relative measurement uncertainty of 4 parts per billion. In 1983, the metre was redefined in the International System of Units (SI) as the distance travelled by light in vacuum in 1⁄299,792,458 of a second. As a result, the numerical value of c in metres per second is now fixed exactly by the definition of the metre.
To help visualise the speed of light, the image below is a scale model of the Earth and the Moon, with a beam of light travelling between them at the speed of light. It takes approximately 1.26 seconds.
Click to view in a new window
*This article and image are licensed under the GNU Free Documentation License. They are sourced from the Wikipedia page for Speed of Light.
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