In the case of nanoscale metal particles, the absorption wavelength associated with the sp transitions depends on the shape and size of the particle. This is a unique property of
nanoparticles that is due to the fact that the s-p (conduction) electrons are largely free to
move about throughout the nanoparticle, and their energies are therefore sensitive to the
shape and size of the box that contains them. For colloidal silver particles, the shape and
size dependence of the absorption wavelength is such that the color of the particles
depends on their preparation, with larger particles and more asymmetric particles
absorbing at longer wavelengths than smaller, more spherical, particles. The longer
wavelengths correspond to lower frequencies, which means that the electrons in a bigger
box oscillate with a lower frequency than those in a smaller box.