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1.2 Fundamental physical constants

1.2.1 Speed of electromagnetic waves

Modern determinations of c, the speed of electromagnetic radiation in vacuo, obtain c through the relation c = vλ, where v is the frequency either of the radiation itself or of an impressed high-frequency modulation, and λ is the corresponding wavelength. Several measurements based on this principle were carried out in the period 1946 to 1957, using microwave radiation or modulated light; the most accurate was a measurement by microwave interferometry (Froome, 1958), which gave the value c = 299 792.50 ± 0.10 km s⁻¹. The development of methods of stabilizing and measuring the frequency of infra-red lasers led to a number of measurements of much higher accuracy, and in 1973 the following value was recommended by the International Committee for Weights and Measures: c = 299 792.458 ± 0.001 km s⁻¹. This value was based on measurements of the frequency and wavelengths of a helium-neon laser radiation, stabilized to an infra-red transition of methane at about 3.39 µm, and was later confirmed by similar work using radiation of about 9.3 µm wavelength from a stabilized carbon dioxide laser (Metrologia, 1974). This same value for c has now been fixed by the definition of the metre adopted by the CGPM in 1984; any changes in the measured value will in future be ascribed to discrepancies between the maintained metre used to make the measurement and the metre of the SI definition, and the maintained metre will be adjusted accordingly; c is now an exact quantity when expressed in SI units.

References

K. D. Froome (1958) Nature, 181 (258).
Metrologia (1974), 10, 9.

B.W.Petley

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