You all know the story. Roy, I don't agree that resonances are poorly understood in material objects that affect the propagation, scattering of, and emission of electromagnetic waves.
Various mechanisms cause resonances in objects subjected to electromagnetic fields.
For example, in spheres there are scattering resonances in the ultraviolet caused by the collective behavior of the cloud of electrons which surround the molecules that make up the particles. This is called a surface plasmon resonance, and it can be moved all the way into the infrared by merely stretching the particle shape from a sphere into a narrow spheroid with the same volume. This shows that the object shape is very important in determining the wavelength at which a given resonance will be found. 11, No. 8, August 1972. Now let me bring all this down to earth for you.