Plasmon
Plasmons are quantum-mechanical density waves in the charge carriers in a substance such as a metal or semiconductor.Light consists of a fluctuating electrical field that generates a fluctuating magnetic field, which generates a fluctuating electric field etc. When the electrical field interacts with a metal, its electrons will be driven back and forth at the frequency of the fluctuation. The result is that the light will be reflected.
However, there is a frequency limit to where the electrons can respond. If the fluctuation is too fast, the electrons can no longer respond. This limit is called the plasmon frequency, beyond which the electron-plasmon interactions result in the absorption of the photon energy. The plasmon frequency is responsible for the color of gold. Because gold reflects low frequency red and yellow light while absorbing blue light the resulting color is yellow.
