Speed of light in air meters per second?

Answer 1

The speed of light (usually denoted c in physics) is a physical constant, the speed at which electromagnetic radiation, such as light, travels in free space (that is, in a perfect vacuum). Its value is 299,792,458 metres per second. This constant is significant in the understanding and study of astronomy, space travel and other fields.

According to the theory of special relativity, c is an important constant connecting space and time in the unified structure of spacetime, and its square is the constant of proportionality between mass and energy (E = mc2).In any inertial frame of reference, independently of the relative velocity of the emitter and the observer, c is the speed of all massless particles and associated fields, including all electromagnetic radiation in free space, and it is believed to be the speed of gravity and of gravitational waves. It is an upper bound on the speed at which energy, matter, and information can travel, as surpassing it would be equivalent to traveling backwards in time; its finite value is a limiting factor in the speed of operation of electronic devices.

For much of human history, it was not known whether light was transmitted instantaneously or simply very quickly. In the 17th century, Ole Rømer first demonstrated that it traveled at a finite speed by studying the apparent motion of Jupiter's moon Io; using these observations, Christiaan Huygens estimated the speed of light to be about 220,000 kilometres per second. Since then, scientists have devised increasingly sophisticated techniques to improve the precision of measurement. By 1975, the speed of light was known to be 299,792,458 m/s with a relative measurement uncertainty of 4 parts per billion (4 × 10−9), mostly due to the uncertainty in the length of the metre. In 1983, the metre was redefined in the International System of Units (SI) as the distance travelled by light in vacuum in 1⁄299,792,458 of a second. As a result, the numerical value of c in metres per second is now a fixed, exact value by definition.

The actual 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 travel at c/1.5 ≈ 200,000 km/s; the refractive index of air is about 1.0003, so the speed of light in air is very close to c.