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From FSU:

“Speed of Light
Somewhere in outer space, billions of light years from Earth, the original light associated with the Big Bang of the universe is blazing new ground as it continues moving outward. In stark contrast, another form of electromagnetic radiation originating on the Earth, radio waves from the inaugural live episode of The Lucy Show are broadcasting a premier somewhere in deep space, although greatly reduced in amplitude.

The basic concept behind both events involves the speed of light (and all other forms of electromagnetic radiation), which scientists have thoroughly examined, and is now expressed as a constant value denoted in equations by the symbol c. Not truly a constant, but rather the maximum speed in a vacuum, the speed of light, which is almost 300,000 kilometers per second, can be manipulated by changing media or with quantum interference.

Light traveling in a uniform substance, or medium, propagates in a straight line at a relatively constant speed, unless it is refracted, reflected, diffracted, or perturbed in some other manner. This well-established scientific fact is not a product of the Atomic Age or even the Renaissance, but was originally promoted by the ancient Greek scholar, Euclid, somewhere around 350 BC in his landmark treatise Optica. However, the intensity of light (and other electromagnetic radiation) is inversely proportional to the square of the distance traveled. Thus, after light has traveled twice a given distance, the intensity drops by a factor of four.

When light traveling through the air enters a different medium, such as glass or water, the speed and wavelength of light are reduced (see Figure 2), although the frequency remains unaltered. Light travels at approximately 300,000 kilometers per second in a vacuum, which has a refractive index of 1.0, but it slows down to 225,000 kilometers per second in water (refractive index of 1.3; see Figure 2) and 200,000 kilometers per second in glass (refractive index of 1.5). In diamond, with a rather high refractive index of 2.4, the speed of light is reduced to a relative crawl (125,000 kilometers per second), being about 60 percent less than its maximum speed in a vacuum.

Because of the enormous journeys that light travels in outer space between galaxies (see Figure 1) and within the Milky Way, the expanse between stars is measured not in kilometers, but rather light-years, the distance light would travel in a year. A light-year equals 9.5 trillion kilometers or about 5.9 trillion miles. The distance from Earth to the next nearest star beyond our sun, Proxima Centauri, is approximately 4.24 light-years. By comparison, the Milky Way galaxy is estimated to be about 150,000 light-years in diameter, and the distance to the Andromeda galaxy is approximately 2.21 million light-years. This means that light leaving the Andromeda galaxy 2.21 million years ago is just arriving at Earth, unless it was waylaid by reflecting celestial bodies or refracting debris.

When astronomers gaze into the night skies, they are observing a mixture of real time, the recent past, and ancient history. For example, during the period that pioneering Babylonians, Arab astrologers, and Greek astronomers described the stellar constellations, Scorpius (Scorpio to astrologers) still had the whiptail of a scorpion. The tail star and others in this constellation had appeared as novae in the skies between 500 and 1000 BC, but are no longer visible to today’s stargazers. Although some of the stars that are observed in the night skies of Earth have long since perished, the light waves that carry their images are still reaching human eyes and telescopes. In effect, the light from their destruction (and the darkness of their absence) has not yet crossed the enormous distances of deep space because of insufficient time.

Empedocles of Acragas, who lived around 450 BC, was one of the first recorded philosophers to speculate that light traveled with a finite velocity. Almost a millennium later, around 525 AD, Roman scholar and mathematician Anicius Boethius attempted to document the speed of light, but after being accused of treason and sorcery, was decapitated for his scientific endeavors. Since the earliest application of black powder for fireworks and signals by the Chinese, man has wondered about the speed of light. With the flash of light and color preceding the explosive sound by several seconds, it did not require a serious calculation to realize that the speed of light obviously exceeded the speed of sound.

http://micro.magnet.fsu.edu/primer/lightandcolor/speedoflight.html