vedphoto | Experimental determination of the speed of light

Experimental determination of the speed of light

Ved Chirayath
Stanford University Department of Physics, Stanford, California 

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.