What is Dipolar Dissipation? When light hits an object, a part of it gets absorbed. Sometimes, people refer to that as "dipolar dissipation". What do they mean exactly by that and does it apply for all materials ? 
Thanks.
 A: Light is associated with an oscillating electric dipole (and an oscillating magnetic dipole though that is not usually relevant for dissipation). That means it exerts an oscillating force on any electrons it shines through, and solids and liquids are full of electrons.
In a transparent material the oscillating electric dipole of the light induces oscillations in the electrons in the dielectric it is passing through. This transfers energy from the light to the electrons. The oscillating electrons then reradiate the light, because any oscillating charge radiates. This transfers the energy back to the light and the end result is that the light passes through the dielectric unchanged.
However there may be other ways for the oscillating electrons to lose energy, and if that happens the energy doesn't get transferred back to the light with the end result that some of the light is absorbed. This is the basis of dipolar dissipation.
An obvious example of this is when the frequency of the light exactly matches some electronic transition in the dielectric. In that case the energy from the light will be efficiently channelled into electronic excitations in the dielectric. The light may be reradiated, e.g. in fluorescence, but most commonly the excited state decays by transferring its energy to the crystal lattice. The end result of this is that the light is absorbed and converted to heat.
But even when the frequency of light does not match an electronic transition there may still some coupling of the electron oscillations to the lattice i.e. some of the energy is channelled into heat rather than being returned to the light. This is usually not a large effect, so the light absorption will not be strong, but it is one of the reasons that no perfectly transparent materials exist.
Metals are a special case since they contain (almost) free electrons, and the free electrons couple very strongly to the oscillating dipole in the light then transfer the energy very efficiently to other electrons and ultimately the lattice. This is why metals are (generally) opaque to all frequencies of light.
