Abstract
THE precision of optical methods of measuring the velocity c of electromagnetic radiation in free space is restricted by the upper limit of the frequency at which a light wave may be amplitude-modulated. Bergstrand1, in his recent measurements, used a modulation frequency of 10 Mc./sec. and consequently his base-lines were several km. long. The highest modulation frequency so far reported seems to be 300 Mc./sec. by Rao and Murty2 using diffraction by ultrasonic waves in a crystal. It would be desirable to increase this limit so that an optical standard of length could be used directly in a determination of c. The correlation of photons in coherent light, which now seems well established3, eliminates the necessity for a direct modulation of a light source. Two wave trains of optical frequencies ν1 and ν2 falling on a photosensitive surface, will give4 a component in the photo-current at a frequency (ν1 − ν2). If the wave trains are divided at a half-silvered mirror and fall on two photocells P 1 and P 2, the outputs S 1 and S 2 of these cells at a frequency (ν1 − ν3) may be added and will interfere to give an output S 3, the amplitude of which depends on the relative phases of the two signals S 1 and S 2. It may readily be shown that if P 1 is kept fixed and P 2 is moved radially away from the source, the amplitude of S 3 will have minima separated by a distance ½c/(ν1 − ν2) if the source can be considered to be a distant point.
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References
Bergstrand, E., Arkiv Fysik, 3, 479 (1951).
Rao, B. R., and Murty, J. S., Nature, 178, 160 (1956).
Twiss, R. Q., Little, A. G., and Hanbury Brown, R., Nature, 180, 324 (1957).
Forrester, A. T., Gudmundsen, R. A., and Johnson, P. O., Phys. Rev., 99, 1691 (1955).
Schawlow, A. L., and Townes, C. H. (to be published).
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SANDERS, J. A Proposed Method for the Measurement of the Velocity of Light. Nature 183, 312 (1959). https://doi.org/10.1038/183312a0
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DOI: https://doi.org/10.1038/183312a0
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