What is the spin of a photon?

The spin of a photon is 1, measured in units of the reduced Planck constant ℏ.

Photons are elementary particles and gauge bosons of the electromagnetic field. Because they are massless spin-1 particles, a freely propagating photon does not display all three spin projections associated with a massive spin-1 particle. Instead, it has two allowed helicity states, commonly written as +1 and −1, corresponding to opposite circular polarizations.

This is a frequent source of confusion. Saying that a photon has spin 1 describes its intrinsic spin representation, while saying that its helicity is +1 or −1 describes the component of angular momentum along its direction of motion. A free photon has no physical longitudinal polarization state, so a spin projection of 0 is not an allowed photon polarization in vacuum.

Photon spin is related to polarization and carries angular momentum. Circularly polarized light transfers angular momentum in units of ℏ per photon, while linearly polarized light can be understood as a superposition of the two opposite-helicity states. Photons are bosons because their spin is integer-valued.