What happens to light of a certain frequency that encounters atoms of the same resonant frequency?
The light would be absorbed, and probably amplified and reemitted (this is how lasers work). A photon (or electron) strikes an atom, which goes from ground state to an excited state – then the atom emits a photon and returns to the ground state. You can make a CO2 gas laser with a 12V 1A power supply, an old ignition coil, a couple of pieces of unetched circuit board, a loaf pan, and some wire. You’ll also need a tank of CO2. You use the power supply to charge the coil. When you pass the current through the CO2, the gas will lase (rather crudely), as the current jumps the gap between the circuit boards across the pan. Scientific American had complete plans for this in one of their issues in the late 70s.
