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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).
At a sufficient distance, the speed at which the beam "moves" may exceed the speed of light. The lighthouse paradox is a thought experiment in which the speed of light is apparently exceeded. The rotating beam of light from a lighthouse is imagined to be swept from one object to shine on a second object. The farther the two objects are away ...
To help compare different orders of magnitude, the following list describes various speed levels between approximately 2.2 × 10 −18 m/s and 3.0 × 10 8 m/s (the speed of light). Values in bold are exact.
The pure number we call the fine-structure constant and denote by α is a combination of the electron charge, e, the speed of light, c, and the Planck constant, h. At first we might be tempted to think that a world in which the speed of light was slower would be a different world. But this would be a mistake.
T 25 is 298.15 K (= 25 °C = 77 °F), giving a value of 346.1 m/s (= 1 135.6 ft/s = 1246 km/h = 774.3 mph = 672.8 kn). In fact, assuming an ideal gas, the speed of sound c depends on temperature and composition only, not on the pressure or density (since these change in lockstep for a given temperature and cancel out). Air is almost an ideal gas.
The two-way speed of light is the average speed of light from one point, such as a source, to a mirror and back again. Because the light starts and finishes in the same place, only one clock is needed to measure the total time; thus, this speed can be experimentally determined independently of any clock synchronization scheme.
In 1845, Arago suggested to Fizeau and Foucault that they attempt to measure the speed of light. Sometime in 1849, however, it appears that the two had a falling out, and they parted ways. [5]: 124 [3] In 1848−49, Fizeau used, not a rotating mirror, but a toothed wheel apparatus to perform an absolute measurement of the speed of light in air.
light-second 1 light-second 299 792 458 m: 2.998 × 10 5 km: 1.863 × 10 5 miles: Average distance from the Earth to the Moon is about 1.282 light-seconds light-minute 60 light-seconds = 1 light-minute 17 987 547 480 m: 1.799 × 10 7 km: 1.118 × 10 7 miles: Average distance from the Earth to the Sun is 8.317 light-minutes light-hour 60 light ...