High Quality Content by WIKIPEDIA articles! The angular momentum of light is a vector quantity that expresses the amount of dynamical rotation present in the electromagnetic field of the light. Indeed, a beam of light, while traveling approximately in a straight line, can also be rotating (or “spinning”, or “twisting”) around its own axis. This rotation, while not visible to the naked eye, can be revealed by the interaction of the light beam with matter. The total angular momentum of light (or, more generally, of the electromagnetic field and the other force fields) and matter is conserved in time. But there are actually two distinct forms of rotation of a light beam, one involving its polarization and the other its wavefront shape. These two forms of rotation are hence associated with two distinct forms of angular momentum, respectively named light spin angular momentum (SAM) and light orbital angular momentum (OAM).