Changes in atoms and in nuclei can generate and absorb radiation. The radiation can come from any part of the electromagnetic spectrum. In order of increasing frequency the spectrum runs: radio waves, microwaves, infrared, visible light, ultraviolet, X-rays and gamma rays.
When electrons in an atom change energy level, they absorb or emit radiation such as infrared, visible light or ultraviolet. The bigger the energy change, the higher the frequency of the radiation. Changes in the nucleus involve much larger energy changes, and a nucleus can emit gamma rays, which have the highest frequency of all.
Fluorescence is an example. A black light lamp gives out ultraviolet radiation. Tonic water absorbs the ultraviolet and then emits visible light, so it glows blue. This happens because of energy changes in the atoms and molecules in the tonic water.