<Tr> <Th> from the Andromeda Galaxy to Earth </Th> <Td> 2.5 million years </Td> </Tr> <Tr> <Th> from Earth to the edge of the observable universe </Th> <Td> 46.5 billion years </Td> </Tr> <P> The speed of light in vacuum, commonly denoted c, is a universal physical constant important in many areas of physics . Its exact value is 7008299792458000000 ♠ 299,792,458 metres per second (approximately 7008300000000000000 ♠ 3.00 × 10 m / s, or 300,000 km / s (186,000 mi / s)). It is exact because the unit of length, the metre, is defined from this constant and the international standard for time . According to special relativity, c is the maximum speed at which all conventional matter and hence all known forms of information in the universe can travel . Though this speed is most commonly associated with light, it is in fact the speed at which all massless particles and changes of the associated fields travel in vacuum (including electromagnetic radiation and gravitational waves). Such particles and waves travel at c regardless of the motion of the source or the inertial reference frame of the observer . In the theory of relativity, c interrelates space and time, and also appears in the famous equation of mass--energy equivalence E = mc . </P> <P> The speed at which light propagates through transparent materials, such as glass or air, is less than c; similarly, the speed of electromagnetic waves in wire cables 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 ≈ 200,000 km / s (124,000 mi / s); the refractive index of air for visible light is about 1.0003, so the speed of light in air is about 299,700 km / s (186,220 mi / s) (about 90 km / s (56 mi / s) slower than c). </P>

What is the speed of light in km