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 [formula omitted]. The accepted value of the speed of light in air to the same number of significant figures is [formula omitted]. © 1969, American Association of Physics Teachers. All rights reserved.



International Standard Serial Number (ISSN)

1943-2909; 0002-9505

Document Type

Article - Journal

Document Version

Final Version

File Type





© 2023 The Authors, All rights reserved.

Publication Date

01 Jan 1969

Included in

Physics Commons