Optical characteristics of manganites with nanoscale electronic phase separation are simulated using an exact renormalization group transformation in Kirchhoff’s equations. The local electric field is found to be highly inhomogeneous, exceeding the incident-wave field by orders of magnitude when the permittivities of the phases have opposite signs and plasmons are excited. The spatial scale of the field fluctuations suggests the collective character of the plasmon modes. The results of the simulation explain the optical anomalies of La_0.7Ca_0.3MnO₃ single crystals and films in the infrared frequency range in a natural way.