All photons, of whatever form (light, infrared warmth, or the eerie whistlers of cosmic rays), are energetic transmissions through the electromagnetic field. That is the field to which charged particles, and only charged particles, are coupled. Charged particles exchange energy in photons, through the field.

Quantum models of the atom enumerate exactly the orbiting levels electrons may "inhabit" around a particular nucleus, or in a molecular array. Moving from one permitted level to a lower one necessarily involves shedding some energy, a precise amount of energy calculated as a difference between the two levels involved. The same quantum of energy, a photon of this frequency, is emitted every time, everywhere, this particular orbital transition takes place. And vice-versa, to get "pumped up" through the same transition, an electron must absorb the same quantum—a photon just the like the one it emitted.

In actuality, a number of possible shifts (a band) are allowable for electrons in whatever atomic or molecular configuration, so that photons of several frequencies may interact with a given electron. Photons that don't match some permitted transition energy for a particular electron in orbit simply won't be noticed, neither emitted nor absorbed.