When a light beam is circularly polarized, each of its photons carries a spin angular momentum (SAM) of ± ± ℏ ℏ , where ℏ ℏ is the reduced Planck constant and the ± ± sign is positive for left and negative for right circular polarizations (this is adopting the convention from the point of view of the receiver most commonly used in optics). This SAM is directed along the beam axis (parallel if positive, antiparallel if negative). The above figure shows the instantaneous structure of the electric field of left ( L ) and right ( R ) circularly polarized light in space. The green arrows indicate the propagation direction.