Mechanical Detection and Measurement of the Angular Momentum of Light

Phys. Rev. 50, 115 – Published 15 July 1936
Richard A. Beth


The electromagnetic theory of the torque exerted by a beam of polarized light on a doubly refracting plate which alters its state of polarization is summarized. The same quantitative result is obtained by assigning an angular momentum of  (-) to each quantum of left (right) circularly polarized light in a vacuum, and assuming the conservation of angular momentum holds at the face of the plate. The apparatus used to detect and measure this effect was designed to enhance the moment of force to be measured by an appropriate arrangement of quartz wave plates, and to reduce interferences. The results of about 120 determinations by two observers working independently show the magnitude and sign of the effect to be correct, and show that it varies as predicted by the theory with each of three experimental variables which could be independently adjusted.


  • Received 8 May 1936
  • Published in the issue dated July 1936

© 1936 The American Physical Society

Authors & Affiliations

Richard A. Beth*

  • Worcester Polytechnic Institute, Worcester, Mass. and Palmer Physical Laboratory, Princeton University

  • *Member of the Physics Department, Worcester Polytechnic Institute. Preliminary work and final calculations were carried on at Worcester. Experimental work was done on leave-of-absence from Worcester as Research Associate at Palmer Physical Laboratory, Princeton University. Additional measurements were made by Mr. W. Harris at Princeton, as will be explained later.

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