  It is shown that the momentum density of free electromagnetic field splits
into two parts. One has no contribution to the net momentum due to the
transversality condition. The other yields all the momentum. The angular
momentum that originates from the former part is spin, and the angular momentum
that originates from the latter part is orbital angular momentum. Expressions
for the spin and orbital angular momentum are given in terms of the electric
vector in reciprocal space. The spin and orbital angular momentum defined this
way are used to investigate the angular momentum of nonparaxial beams that are
described in a recently published paper [Phys. Rev. A 78, 063831 (2008)]. It is
found that the orbital angular momentum depends, apart from an $l$-dependent
term, on two global quantities, the polarization represented by a generalized
Jones vector and a new characteristic represented by a unit vector
$\mathbf{I}$, though the spin depends only on the polarization. The
polarization dependence of orbital angular momentum through the effect of
$\mathbf{I}$ is obtained and discussed. Some applications of the result
obtained here are also made. The fact that the spin originates from the part of
momentum density that has no contribution to the net momentum is used to show
that there does not exist the paradox on the spin of circularly polarized plane
wave. The polarization dependence of both spin and orbital angular momentum is
shown to be the origin of conversion from the spin of a paraxial
Laguerre-Gaussian beam into the orbital angular momentum of the focused beam
through a high numerical aperture.
