# Speed of light

The speed at which light propagates through transparent materials, such as glass or air, is less than c. The ratio between c and the speed v at which light travels in a material is called the refractive index n of the material (n = c / v). For example, for visible light the refractive index of glass is typically around 1.5, meaning that light in glass travels at c / 1.5 ≈ 200000 km/s; the refractive index of air for visible light is 1.000293, so the speed of light in air is 299705 km/s or about 88 km/s slower than c. In most practical cases, light and other electromagnetic waves can be thought of as moving "instantaneously", but for long distances and very sensitive measurements their finite speed has noticeable effects. Numerical value, notation, and units The speed of light in vacuum is usually denoted by c, for "constant" or the Latin celeritas (meaning "swiftness"). Fundamental role in physics The Lorentz factor γ as a function of velocity. Upper limit on speeds

Natural resources The Earth's natural resources are vital to the survival and development of the human population. These resources are limited by the Earth's capability to renew them. Although many effects of overexploitation are felt locally, the growing interdependence of nations, and international trade in natural resources, make their demand and sustainable management a global issue. Light-Speed Constant C The speed of light in a vacuum is 186,282 miles per second (299,792 kilometers per second), and in theory nothing can travel faster than light. In miles per hour, light speed is, well, a lot: about 670,616,629 mph. If you could travel at the speed of light, you could go around the Earth 7.5 times in one second. Early scientists, unable to perceive light’s motion, thought it must travel instantaneously. Over time, however, measurements of the motion of these wave-like particles became more and more precise.

How Space Shuttles Work" In its nearly 30-year history, the space shuttle program has seen exhilarating highs and devastating lows. The fleet has taken astronauts on dozens of successful missions­, resulting in immeasurable scientific gains. But this success has had a serious cost. In 1986, the Challenger exploded during launch. In 2003, the Columbia broke up during re-entry over Texas. Since the Columbia accident, the shuttles have been grounded pending redesigns to improve their safety. Through The Wormhole: Can We Travel Faster Than Light? Prior to the day in 1947 when test pilot Charles E. Chuck Yeager broke the sound barrier for the first time, people argued it wasn't possible for a plane to fly that fast. So, perhaps we shouldn't be deterred by the part of Einstein's special theory of relativity that seemingly bars traveling at speeds faster than light. That said, cracking the light-speed barrier is vastly more complicated than going faster than sound. The aircraft that Yeager used to break Mach 1, for example, didn't have to change form.

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