Speed of light

The speed of light in a vacuum is defined to be 299,792,458 metres per second (1,079,252,848.8 km/h, which is approximately 186,282.4 miles per second, or 670,616,629.38 miles per hour). The speed of light is denoted by the letter c, reputedly from the Latin celeritas, "speed", and also known as Einstein's constant. Note that this speed is a definition, not a measurement; in fact, the fundamental SI unit of distance, the metre, is defined in terms of the speed of light and the second. The speed of light through a transparent medium (that is, not in vacuum) is less than c; the ratio of c to this speed is called the refractive index of the medium. "Speed of light" is sometimes abbreviated SOL.

External links

• speed of light in vacuum (at NIST)
• A Brief History of c
• Data Gallery: Michelson Speed of Light (Univariate Location Estimation) (download data gathered by A.A. Michelson)
• Switching light on and off (news article on stopping light)
• Beam smashes light barrier (news article on group velocity experiment)
• Subluminal (Java applet demonstrating group velocity information limits)
• Light discussion on adding velocities

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