  We introduce a simple theoretical model that describes the interaction of
light with optical metamaterials in terms of interfering optical plane waves.
In this model, a metamaterial is considered to consist of planar arrays of
densely packed nanoparticles. In the analysis, each such array reduces to an
infinitely thin homogeneous sheet. The transmission and reflection coefficients
of this sheet are found to be equal to those of an isolated nanoparticle array
and, therefore, they are easy to evaluate numerically for arbitrary shapes and
arrangements of the particles. The presented theory enables fast calculation of
electromagnetic fields interacting with a metamaterial slab of an arbitrary
size, which, for example, can be used to retrieve the effective refractive
index and wave impedance in the material. It is also shown to accurately
describe optically anisotropic metamaterials that in addition exhibit strong
spatial dispersion, such as bifacial metamaterials.
