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
Speed of light for interferometry measurements of the speed of light. However interferometric determination of wavelength becomes less precise with wavelength and the experiments were thus limited in precision by the long wavelength (~) of the radiowaves. The precision can be improved by using light with a shorter wavelength, but then it becomes difficult to directly measure the frequency of the light. One way around this problem is to start with a low frequency signal of which the frequency can be precisely measured, and from this signal progressively synthesize higher frequency signals whose frequency can then be linked to the original signal. A