Lycurgus Cage Cup

This example is quite often used by physicists as an example for the effect of particle size on absorption/transmission characteristics. In this case the material contains nm-sized gold particles that have a so called 'plasmon resonance' at a certain frequency. All the electrons wiggle at their best when this takes place. Take a pendulum as the mechanical analogy. If you excite it with its natural frequency (eigen-frequency), the amplitude is greatest.