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Propagation of light

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In classical physics, light is described as a type of electromagnetic wave. The classical behaviour of the electromagnetic field is described by Maxwell's equations, which predict that the speed  c with which electromagnetic waves (such as light) propagate in vacuum is related to the distributed capacitance and inductance of vacuum, otherwise respectively known as the electric constant ε 0 and the magnetic constant μ 0 , by the equation c = 1 ε 0 μ 0   . {\displaystyle c={\frac {1}{\sqrt {\varepsilon _{0}\mu _{0}}}}\ .} In modern quantum physics, the electromagnetic field is described by the theory of quantum electrodynamics (QED). In this theory, light is described by the fundamental excitations (or quanta) of the electromagnetic field, called photons. In QED, photons are massless particles and thus, according to special relativity, they travel at the speed of light in vacuum. Extensions of QED in which the photon has a mass have been considered. In...
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Faster-than-light observations and experiments

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There are situations in which it may seem that matter, energy, or information travels at speeds greater than  c , but they do not. For example, as is discussed in the propagation of light in a medium section below, many wave velocities can exceed  c . For example, the phase velocity of X-rays through most glasses can routinely exceed c , but phase velocity does not determine the velocity at which waves convey information. If a laser beam is swept quickly across a distant object, the spot of light can move faster than  c , although the initial movement of the spot is delayed because of the time it takes light to get to the distant object at the speed  c . However, the only physical entities that are moving are the laser and its emitted light, which travels at the speed  c from the laser to the various positions of the spot. Similarly, a shadow projected onto a distant object can be made to move faster than  c , after a delay in time. In neither case does any matter, energy, or informat...
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Measurement

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There are different ways to determine the value of c . One way is to measure the actual speed at which light waves propagate, which can be done in various astronomical and earth-based setups. However, it is also possible to determine c from other physical laws where it appears, for example, by determining the values of the electromagnetic constants ε 0 and μ 0 and using their relation to c . Historically, the most accurate results have been obtained by separately determining the frequency and wavelength of a light beam, with their product equalling c . In 1983 the metre was defined as "the length of the path travelled by light in vacuum during a time interval of ​ 1⁄ 299 792 458 of a second", fixing the value of the speed of light at 299 792 458  m/s by definition, as described below. Consequently, accurate measurements of the speed of light yield an accurate realization of the metre rather than an accurate value of c . Astronomical measurements Outer space is a convenie...
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