Experimental determination of the speed of light

By exploiting the resonance condition of an open cavity He-Ne laser, we measure the variation between longitudinal mode frequencies, or beat frequencies, as a function of cavity length. Using a high frequency photodiode and radio spectrum analyzer, we empirically determine the speed of light as 𝑐 = 299.55 7 x 10^6  m/s - within 0.79% of the defined value. The measurement of the speed of light is fundamental to the realization of both a physical constant – the speed at which electromagnetic radiation propagates - and the standard unit of length from which it is defined, the meter.