  We propose a new approach to theoretical description of doped cuprate like
$La_{2-x}Sr_{x}CuO_4$ and $YBa_2Cu_3O_{6+x}$, assuming phase separation and
treating it as inhomogeneous composite material, containing the dielectric and
metallic stripe-like nanoparticles. The formalism of effective medium theory is
then applied for calculation of dielectric permittivity, optical and EELS
spectra of $La_{2-x}Sr_{x}CuO_4$ with $x$ varying in a wide range. Reasonable
semi-quantitative agreement with experiment has been obtained even for the
simplest version of the theory. The model was found able to reproduce all
essential features of optical conductivity $\sigma(\omega)$ and transmittance
both for thin films (M. Suzuki, Phys. Rev. B 39, 2321 (1989)) and bulk
single-crystalline samples (S. Uchida {\it et al.} Phys. Rev. B 43, 7942
(1991)). Substantial difference in spectral and doping dependence of optical
absorption for the thin-film and bulk samples is easily explained if only to
assume different shape of metallic and dielectric regions in both materials.
New peaks in optical conductivity and absorption spectra, that emerge in the
midinfrared range near 0.5 and 1.5 eV upon doping are attributed to geometrical
(Mie's) resonances. Overall, we point out that all main peculiarities of the
doping effect on optical and EELS spectra for cuprates including the spectral
changes accompanying the insulator-to-metal transition can be explained rather
prosaically by recognizing that the doping results in emergence of nanoscopic
metallic stripe-like droplets.
