Absorption spectra from colloids containing different concentrations of spherical gold nanoparticles in water were measured with a spectrophotometer. The absorption spectra were used to calculate the number density of nanoparticles (NPs) with the aid of an unconventional finite spectral band <i>f</i>-sum rule applied for gold colloid. Good correlation between the number density of dispersion electrons, obtained from the <i>f</i>-sum rule, and the number density of nanoparticles was found. The effective absolute refractive index of the gold colloid was obtained with the aid of a singly subtractive Kramers–Kronig relation, and in addition the refractive index change due to the nanoparticles was obtained with the aid of a conventional Kramers–Kronig relation. Such optical properties are valuable in studies of light interaction with nanoparticles.
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