Final answer:
A material like CsI that fluoresces when stimulated by radiation emits light as a result of energy absorption causing electrons to jump and then fall back to lower energy states, releasing photons.
Step-by-step explanation:
A material that fluoresces, such as CsI (Cesium Iodide), when stimulated by radiation emits light. This phenomenon occurs as the absorption of energy causes an electron to jump to a higher energy state. Shortly afterward, the electron falls back to its ground state, releasing a photon of light in the process. The emitted photon is of lower energy and frequency compared to the absorbed radiation given the energy initially absorbed. Materials like CsI that exhibit this property are known as scintillators and are used in various radiation detection applications, including research and diagnostics.