speed of light is the speed of electromagnetic waves. Light travels at slower speed inside materials like glass or water; its highest speed is in a vacuum The speed of light is the speed of electromagnetic waves. Light travels at slower speed inside materials like glass or water; its highest speed is in a vacuum. The speed of light in vacuum is a universal physical constant denoted c (in ISO and IEC standards c 0 {\displaystyle c_{0}} ), exactly equal t The speed of light is the speed of electromagnetic waves. Light travels at slower speed inside materials like glass or water; its highest speed is in a vacuum. The speed of light in vacuum is a universal physical constant denoted c (in ISO and IEC standards c 0 {\displaystyle c_{0}} ), exactly equal to 299792458 m⋅s−1. It is exact because, by international agreement, a metre is defined as the length of the path travelled by light in vacuum during a time interval of 1⁄299792458 second. The value 299,792,458 metres per second is approximately 1 billion kilometres per hour; 700 million miles per hour. For other approximations of c, valid for various units and size scales see the infobox. The shorter form, speed of light, means speed of light in vacuum in any context where the vacuum is implied. All forms of electromagnetic radiation, including visible light, travel in vacuum at the speed c as do massless particles and field perturbations, such as gravitational waves. The speed of light in vacuum is the same for all observers, no matter their relative velocity. As a result, massless particles and waves travel at c in a vacuum regardless of the motion of the source or the inertial reference frame of the observer. The speed of light in vacuum is the upper limit for the speed at which information, matter, or energy can travel thr The speed of light is the speed of electromagnetic waves. Light travels at slower speed inside materials like glass or water; its highest speed is in a vacuum. The speed of light in vacuum is a universal physical constant denoted c (in ISO and IEC standards c 0 {\displaystyle c_{0}} ), exactly equal to 299792458 m⋅s−1. It is exact because, by international agreement, a metre is defined as the length of the path travelled by light in vacuum during a time interval of 1⁄299792458 second. The value 299,792,458 metres per second is approximately 1 billion kilometres per hour; 700 million miles per hour. For other approximations of c, valid for various units and size scales see the infobox. The shorter form, speed of light, means speed of light in vacuum in any context where the vacuum is implied. All forms of electromagnetic radiation, including visible light, travel in vacuum at the speed c as do massless particles and field perturbations, such as gravitational waves. The speed of light in vacuum is the same for all observers, no matter their relative velocity. As a result, massless particles and waves travel at c in a vacuum regardless of the motion of the source or the inertial reference frame of the observer. The speed of light in vacuum is the upper limit for the speed at which information, matter, or energy can travel through space. Particles with nonzero rest mass can be accelerated to approach c but can never reach it, regardless of the frame of reference in which their speed is measured. For long distances and sensitive measurements, the finite speed of light has noticeable effects. Much starlight viewed on Earth is from the distant past, allowing humans to study the history of the universe by viewing distant objects. When communicating with distant space probes, it can take hours for signals to travel. In computing, the speed of light fixes the ultimate minimum communication delay. The speed of light in vacuum can be used in time of flight measurements to measure large distances to extremely high precision. The speed at which light propagates through transparent materials, such as glass or air, is less than c; similarly, the speed of electromagnetic waves around wire cables (the speed of electricity) is slower 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 travels at ⁠c/1.5⁠ ≈ 200000 km/s (124000 mi/s); the refractive index of air for visible light is about 1.0003, so the speed of light in air is about 90 km/s (56 mi/s) slower than c. In the context of relativity, luminal speed may be used. In some cases, objects or waves may appear to travel faster than light, also called superluminal speed. Simplistic calculations of the speed of distant galaxies due to the expansion of the universe appear to exceed the speed of light, but only local velocity can be defined uniquely. Ole Rømer first demonstrated that light does not travel instantaneously by studying the apparent motion of Jupiter's moon Io. In an 1865 paper, James Clerk Maxwell proposed that light was an electromagnetic wave and, therefore, travelled at speed c. Albert Einstein postulated that the speed of light c with respect to any inertial frame of reference is a constant and is independent of the motion of the light source. He explored the consequences of that postulate by deriving the theory of relativity, and so showed that the parameter c had relevance outside of the context of light and electromagnetism. In the theory of relativity, c interrelates space and time and appears in the famous mass–energy equivalence, E = mc2. None of these options allow information to be transmitted faster than c. It is impossible to transmit information with a light pulse any faster than the speed of the earliest part of the pulse (the front