J ( x ) = 2 1 − − x cot ⁡ ⁡ ( x ) x 2 + x cot ⁡ ⁡ ( x ) − − 1 , where a is the nanoparticle radius and k m = ε ε m μ μ m ω ω / c is wave number. It is supposed here that the time dependence of the electromagnetic field is given by the factor e x p ( − − i ω ω t ) . In this paper Bruggeman's approach was used, but electromagnetic field for electric-dipole oscillation mode inside the picked particle was computed without applying quasi-static approximation. Thus the function J ( k m a ) is due to the field nonuniformity inside the picked particle. In quasi-static region ( k m a ≪ ≪ 1 , i.e. a ≤ ≤ 10 n m for Ag ) this function becomes constant J ( k m a ) = 1 and formula (5) becomes identical with Bruggeman's formula.