light

See also: color temperature; color rendering index.

Velocity of light

Since 1983, the velocity of light in a vacuum has been 299,792,458 meters per second.  No experimental results can alter this value. It is a matter of definition, because in that year the CGPM redefined¹ the meter to base it on the velocity of light, rather than on a count of wavelengths of light emitted by krypton-86 atoms, or, as it was even earlier, on marks on a metal bar, the International Prototype of the Meter. See meter. In physics, the symbol for the velocity of light is “c”.

The velocity of light is constant in the sense that, unlike the velocity of moving objects, it is independent of the observer's inertial frame of reference, a perennially astonishing insight famously owed to Albert Einstein. But the velocity does vary with the medium through which the light is passing; it is faster in a vacuum than, say, in water, glass or air. The functionality of lenses depends on this variation.

The velocity of light also varies with the velocity of the medium through which it is traveling. If a person walks at a velocity of 4 miles per hour up the aisle of an airplane traveling 300 miles per hour, his forward velocity is 304 miles per hour. But light does not behave this way! A. H. L. Fizeau (1819–1896) demonstrated that the velocity of light and the medium don't simply add; they are related thus: 

An equation. The velocity of light in the medium is equal to c over n, plus the velocity of the medium times the quantity 1 plus 1 over n squared.,

where n is the refractive index of the medium and vm its velocity.

In a rather extraordinary feat, beginning in the late 1990's researchers were able to slow a pulse of light to a crawl, and even stop and restart it without destroying it².

1. 17th CGPM, Resolution 1.

2. Peter Weiss.
Light stands still in atom clouds.
Science News, volume 159, number 4 (27 January 2001).

Chien Liu, Zachary Dutton, Cyrus H. Behroozi, and Lene Vestergaard Hau.
Observation of coherent optical information storage in an atomic medium using halted light pulses.
Nature, volume 409, pages 490-493 (25 January 2001).

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