![]() 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. 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. The distance from Earth to the next nearest star beyond our sun, Proxima Centauri, is approximately 4.24 light-years. A light-year equals 9.5 trillion kilometers or about 5.9 trillion miles. In diamond, with a relatively 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.īecause of the enormous journeys that light travels in outer space between galaxies 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. 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 = 1.3 see Figure 1) and 200,000 kilometers per second in glass (refractive index of 1.5). 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 1), although the frequency remains unaltered. Thus, after light has traveled twice a given distance, the intensity drops by a factor of four. However, the intensity of light (and other electromagnetic radiation) is inversely proportional to the square of the distance traveled. 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. 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. In addition, the speed of light through a material of the refractive index set by the slider is calculated and displayed beneath the block. The refractive indices and names corresponding to common materials are displayed above the slider handle. As the slider is translated to the right, the refractive index increases and the speed of light subsequently decreases. ![]() In order to operate the tutorial, use the Refractive Index of Material slider to change the composition and refractive index of the block between a range of 1.0 and 3.91. The tutorial initializes with a light wave traveling through air (simulated by a clear block) at a speed of 186,226.52 miles per second. This interactive tutorial explores the reduction in the speed of light as a function of refractive index in common substances. When light traveling in a vacuum enters a new transparent medium, such as air, water, or glass, the speed is reduced in proportion to the refractive index of the new material. Speed of Light in Transparent Materials - Java Tutorial
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