Abstract

Measurements of the radiation torque on a birefringent plate¹ provided early macroscopic evidence that the angular momentum per photon in a pure circularly polarized state is ℏ. Recently, the spin-up of small particles illuminated by circularly polarized light was observed.² Spin-up was apparently due to radiation torques; however, details of the experiment were not given. We calculate the torque on isotropic spheres illuminated by a circularly polarized plane wave. The formulation of the other fundamental cross sections by Mie (1908) and Debye (1909) for homogeneous isotropic spheres give the well-known efficiency factors³ for scattering, absorption, extinction, and radiation pressure usually designated by Q_sca, Q_abs. Q_ext, and Q_pr, respectively.

© 1984 Optical Society of America