When a fluorescent compound is exposed to ultraviolet (UV) light, it absorbs the high-energy UV photons. This absorption promotes an electron within the compound to a higher energy level, referred to as an excited state. The electron in this excited state is unstable and quickly returns to its ground state, releasing the excess energy it gained in the form of light.
The emitted light from a fluorescent compound typically occurs in the visible spectrum rather than the UV spectrum. This is because the absorbed energy causes the compound to undergo a transition between electronic energy levels, and the energy difference between these levels corresponds to the wavelength of light emitted. In the case of fluorescent compounds, this energy difference usually falls within the visible range, resulting in the emission of visible light.
The rest of the energy that the compound absorbed from the UV light is dissipated as heat. Some energy may also be lost through other non-radiative processes, such as internal conversion or vibrational relaxation, which do not result in the emission of light. However, the majority of the absorbed energy is ultimately released as visible light, making the fluorescent compound visible under UV illumination.