Speed of light dimensions of a capacitor, whereas the value of the vacuum permeability is fixed at exactly through the definition of the ampere. Rosa and Dorsey used this method in 1907 to find a value of . Another way to measure the speed of light is to independently measure the frequency "f" and wavelength "λ" of an electromagnetic wave in vacuum. The value of "c" can then be found by using the relation "c" = "fλ". One option is to measure the resonance frequency of a cavity resonator. If the dimensions of the resonance cavity are also known, these can be used
Rømer's determination of the speed of light Rømer's determination of the speed of light Rømer's determination of the speed of light was the demonstration in 1676 that light has a finite speed, and so does not travel instantaneously. The discovery is usually attributed to Danish astronomer Ole Rømer (1644–1710), who was working at the Royal Observatory in Paris at the time. By timing the eclipses of the Jupiter moon Io, Rømer estimated that light would take about 22 minutes to travel a distance equal to the diameter of Earth's orbit around the Sun. This would give light a velocity of about 220,000 kilometres per second, about 26%