  We report numerical experiments of optical wave propagation in composites of
high refractive index dielectric rods at frequencies where their first electric
and magnetic Mie resonances are excited. The arrays of these particles have
been extensively studied and proposed as non-absorbing and isotropic
metamaterials. We show that negative refraction, observed in ordered particle
arrays, is due to diffraction and that an effective medium theory yields
constitutive parameters that do not reproduce the observations in these
composites, whose transmission also depends on the sample shape. This is
further confirmed by disordering the arrays, a case in which large transmission
losses appear due to extinction by resonant scattering from the particles.
Therefore, these composites although little absorbing have large extinction due
to scattering.
