Optical torques align and rotate microscopic wave plates

Summary form only given.The angular momentum carried by polarized light was first measured in 1936 by Richard Beth, who measured the tiny reaction torque due to the change in polarization of light on passage through a quartz wave plate. Using the modern single-beam gradient optical trap, we can observe the reaction torque on a birefringent particle on a microscopic scale, where the observable effects are several orders of magnitude greater. We used trapped calcite fragments dispersed in water in a single beam gradient trap.