Final answer:
A molecule undergoing fluorescence absorbs light and emits it at longer wavelengths due to the release of lower-energy photons during its return to the ground state from an excited state.
Step-by-step explanation:
A chemical molecule that absorbs light and subsequently releases a portion of the energy in photons of longer wavelengths is undergoing a process known as fluorescence. In fluorescence, the absorbed light usually comes from high-energy photons, such as those in the UV range, which causes the molecule to enter an excited state. The molecule then emits light that has less energy and, consequently, a longer wavelength than the absorbed light. This emitted light is often in the visible range.
For instance, when fluorescent compounds like heterocyclic aromatic molecules absorb light, they reach an excited state several steps above their ground state. The subsequent emission of lower-energy photons, as the compounds return to ground state in smaller steps, is the essence of the fluorescence phenomenon.