"The mean free path in gold and silver is ~50 nm, therefore in particles smaller than this, no scattering is expected from the bulk. Thus, all interactions are expected to be with the surface. When the wavelength of light is much larger than the nanoparticle size it can set up standing resonance conditions as represented in Fig. 3. Light in resonance with the surface plasmon oscillation causes the free-electrons in the metal to oscillate. As the wave front of the light passes, the electron density in the particle is polarized to one surface and oscillates in resonance with the light’s frequency causing a standing oscillation."

Link to the original article: Article Why Gold Nanoparticles Are More Precious than Pretty Gold: N...

I have also attached a pdf file of the same.