Scattering and Absorption
At its most basic, the interaction of light with matter entails the interaction of a single atom with a single quantum of light, called a photon. When an atom interacts with a photon, one of two things happen: it either absorbs (and later re-emits) the photon or it scatters the photon. For the atom to absorb the photon, the energy of the photon must exactly match the gap between two of the energy states of one of the atom’s electrons (two so-called “electronic states”). This is what is called an “electronic transition” and is usually between the ground state and an excited state of the atom. This same condition is not true of scattering: the photon may be scattered regardless of its energy (which is directly proportional to its frequency), although different energies will lead to different types of scattering. If the energy of the incident photon is high enough, it can knock an electron completely out of an atom, thereby ionising it.
At its most complicated, the interaction of light with matter entails many photons interacting not only with atoms, but with molecules and agglomerations of molecules and atoms, be they in solid, liquid, or gas form. The photons may be absorbed or scattered, or they may not interact with the material and pass straight through it.
Similarly to an atom, a molecule will only absorb a photon and be promoted to an excited state if the energy of the incident photon corresponds to the gap between the ground state and an excited state of the molecule. However, the gaps between the ground and excited states of a molecule are more complicated than the electronic transitions in an atom, as as a molecule will have not only electronic but also vibrational and rotational sub-levels. In other words, a molecule is composed of two or more atoms, each with their own electronic states, which rotate and vibrate with respect to each other such that the energy stored in the molecule is a sum of electronic, rotational and vibrational energy. As was also the case with an atom, it is possible for a photon to interact with a molecule and scatter from it, with no need for the photon to have an energy which matches the difference between two energy levels of the molecule.