  The measurement of the spin angular momentum of circularly polarized light by
Beth [Phys. Rev. 50, 115 (1936)] can be explained by using a microscopic torque
density. However, the experiment does not resolve the space- and time-dependent
evolution of the spin density of light and the wave plate and the covariant
form of the microscopic torque density. Here we focus on the covariant
description of the helicity, spin, and torque densities of light in materials
using the chiral-stress-energy-momentum tensor. We also perform simulations of
Gaussian light pulses in quarter-wave-plate geometries made of birefringent and
dielectric materials.
