Speed of light fibre optic signals which travel 30–40% slower at the speed of light through glass. There are different ways to determine the value of "c". One way is to measure the actual speed at which light waves propagate, which can be done in various astronomical and earth-based setups. However, it is also possible to determine "c" from other physical laws where it appears, for example, by determining the values of the electromagnetic constants "ε" and "μ" and using their relation to "c". Historically, the most accurate results have been obtained by separately determining the frequency and wavelength of a light beam,
Speed of light This is one of the proposed changes to be incorporated in the next revision of the SI also termed the "New SI". Speed of light The speed of light in vacuum, commonly denoted , is a universal physical constant important in many areas of physics. Its exact value is (approximately ). It is exact because by international agreement a metre is defined to be the length of the path travelled by light in vacuum during a time interval of 1/299792458 second. According to special relativity, is the maximum speed at which all conventional matter and hence all known forms of