|MadSci Network: Physics|
All substances are made of molecules. Electrons in molecules, due to quantum effects, are confined to some discrete energy levels (potential + kinetic). This means that if you want to make an electron jump to a higher level of energy you must supply it with exactly the difference of energy between the final and the initial level.
A wave is a method of transporting energy. It consists of "packets of energy". For an electromagnetic wave this packets are called photons. For mechanical waves, they are called phonons. Radio waves are a form of electromagnetic waves.
The light has exactly the right energy to be "absorbed" by substances around us (most of them). Radio waves however have a much lower energy per packet ratio so there are almost no substances which "absorb" them. Receptors of radio waves are materials that absorb them: they are conductors. These materials, due to their special internal structures have very dense discrete energy levels. Practically you can take these levels to be continuos; that is an electron in a conductor can have any energy within a band (between two frequencies).
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