A Proposed Method for the Measurement of the Velocity of Light

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.