  It is shown that geometric optical description of electromagnetic wave with
account of its polarization in curved space-time can be obtained
straightforwardly from the classical variational principle for electromagnetic
field. For this end the entire functional space of electromagnetic fields must
be reduced to its subspace of locally plane monochromatic waves. We have
formulated the constraints under which the entire functional space of
electromagnetic fields reduces to its subspace of locally plane monochromatic
waves. These constraints introduce variables of another kind which specify a
field of local frames associated to the wave and contain some congruence of
null-curves. The Lagrangian for constrained electromagnetic field contains
variables of two kinds, namely, a congruence of null-curves and the field
itself. This yields two kinds of Euler-Lagrange equations. Equations of first
kind are trivial due to the constraints imposed. Variation of the curves yields
the Papapetrou equations for a classical massless particle with helicity 1.
