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The speed of light in vacuum, commonly denoted c, is a universal physical constant that is exactly equal to 299,792,458 metres per second (approximately 300,000 kilometres per second; 186,000 miles per second; 671 million miles per hour).
where c is the defined value for the speed of light in classical vacuum in SI units, [4]: 127 and μ 0 is the parameter that international standards organizations refer to as the magnetic constant (also called vacuum permeability or the permeability of free space).
Electromagnetic radiation travels, when unobstructed, at the speed of light, the defined value 299,792,458 m/s in SI units. [27] The superposition principle is always exactly true. [28] For example, the electric potential generated by two charges is the simple addition of the potentials generated by each charge in isolation.
The absolute refractive index n of an optical medium is defined as the ratio of the speed of light in vacuum, c = 299 792 458 m/s, and the phase velocity v of light in the medium, =. Since c is constant, n is inversely proportional to v : n ∝ 1 v . {\displaystyle n\propto {\frac {1}{v}}.}
c is the speed of light in free space, µ 0 is the vacuum permeability . The constants c and µ 0 were both defined in SI units to have exact numerical values until the 2019 revision of the SI .
is the speed of light (i.e. phase velocity) in a medium with permeability μ, and permittivity ε, and ∇ 2 is the Laplace operator. In a vacuum, v ph = c 0 = 299 792 458 m/s, a fundamental physical constant. [1] The electromagnetic wave equation derives from Maxwell's equations.
is the speed of light in vacuum. The symbol means that the nucleus is assumed to be infinitely heavy, an improvement of the value can be made using the reduced mass of the atom: = + with the mass of the nucleus. The corrected Rydberg constant is:
The speed of light in vacuum is defined to be exactly 299 792 458 m/s (approximately 186,282 miles per second). The fixed value of the speed of light in SI units results from the fact that the metre is now defined in terms of the speed of light. All forms of electromagnetic radiation move at exactly this same speed in vacuum.