Abstract
A low-frequency, phase-shift method for the measurement of the speed of light has been developed. This technique gives results commensurate with other advanced laboratory methods. One advantage of this technique is that the apparatus is of reasonable size and most of the circuitry involves widely known amateur radio techniques. Furthermore, our use of a low modulating frequency permits use of the solid-state, electro-optical light shutter. This eliminates the rather dangerous liquid Kerr cell, thus making our apparatus more acceptable to application in the undergraduate advanced laboratory. From a pedagogical point of view, the student is allowed to use and become acquainted with the lock-in amplifier which is so commonly found in the modern research laboratory. Two rather novel techniques were employed in this apparatus. These were phase multiplication and mixing of the rf signal in the last stages of the photomultiplier. Our value for the speed of light in air was 2.937 × 108 m /sec. The accepted value of the speed of light in air to the same number of significant figures is 2.997 × 108 m /sec.
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