  A three-orbital-model approach for studying spin wave excitations in the
strongly spin-orbit coupled layered perovskite iridates is presented which
provides a unified description of magnetic excitations as well as the
electronic band structure. The calculated spin wave dispersions with realistic
three-band parameters are in excellent agreement with the RIXS data for
iridates, including the strong AF zone boundary dispersion in the single-layer
compound $\rm Sr_2 Ir O_4$ and the large anisotropy gap in the bilayer compound
$\rm Sr_3 Ir_2 O_7$. The RIXS spin wave data is shown to provide evidence of
mixing between the J=1/2 and 3/2 sectors in both compounds.
