What is the speed of light?

the speed of light = 299 792 458 m / s

The speed of light in the vacuum of free space is an important physical constant usually denoted by the letter c.^[1] It is the speed of all electromagnetic radiation, including visible light, in free space. It is the speed of anything having zero rest mass.^[2] The SI metre is defined such that the speed of light in a vacuum is exactly 299,792,458 metres per second (1,079,252,849 km/h).^[3] The speed of light can be assigned a definite numerical value because the fundamental SI unit of length, the metre, has been defined since October 21, 1983, as the distance light travels in a vacuum in 1/299,792,458 of a second; in other words, any increase in the measurement precision of the speed of light would refine the definition of the metre, but not alter the numerical value of c. The approximate value of 3×10⁸ m/s is commonly used in rough estimates (the error is 0.07%). In imperial units, the speed of light is about 670,616,629.4 miles per hour or 983,571,056.4 feet per second (roughly one foot per nanosecond), which is about 186,282.397 miles per second.

The speed of light when it passes through a transparent or translucent material medium, like glass or air, is less than its speed in a vacuum. The ratio of the speed of light in the vacuum to the observed phase velocity is called the refractive index of the medium. See dispersion (optics). In general relativity c remains an important constant of spacetime, however the concepts of 'distance', 'time', and therefore 'speed' are not always unambiguously defined due to the curvature of spacetime caused by gravitation. When measured locally, light in a vacuum always passes an observer at c.

 

A line showing the speed of light on a scale model of Earth and the Moon, taking about 1⅓ seconds to traverse that distance

When we deal with the immense distances of space, though, even light seems slow. When astronauts were on the Moon it took over a second for the radio waves (which travel at the speed of light) carrying their voices to reach us. Light coming from the sun takes eight and one half minutes to hit Earth. (This means that if the sun were suddenly to go dark, it would take over eight minute for us to notice) Light from the nearest stars, other than the sun, takes four and a half years to get here. From the farthest stars in distant galaxies it can take billions of years for the light to arrive..

Light years

Astronomical distances are sometimes measured in light years (the distance that light would travel in one Earth year, roughly 9.46×10¹² kilometres or about 5.88×10¹² miles). Because light travels at a large but finite speed, it takes time for light to cover large distances. Thus, the light we observe from distant objects in the universe was emitted from them long ago: in effect, we see their distant past. Even in terms of our own star we see into the past as well. Light from the sun takes around eight and one-third minutes to reach the earth.