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We empirically determined the speed of light by measuring the variation inlongitudinal mode frequencies, or the beat frequencies, of anadjustable-length, open-cavity helium-neon laser as a function of its cavitylength. The TEM$_{00}$ mode lasing output of the laser was analyzed using afast frequency photodiode detector and a radio frequency spectrum analyzer. AFabry-Perot interferometer was used to monitor the intensity of thelongitudinal modes and we found that the phenomena of frequency pushing andpulling had little effects on the beat frequency measurements. Plotting thereciprocal of the beat frequency as a function of the change in cavity length,the speed of light was found, by using linear weighted least squaresregression, to be $(2.997 \pm 0.003) \times 10^{8}$ ms$^{-1}$. This value is$0.3 \sigma$ away from the defined value of speed of light and is accurate to 1part in 3200.

Determination of the speed of light from the longitudinal modes beat frequencies of an open-cavity helium-neon laser