Fluorescence is a characteristic phenomenon in which solids, liquids, or gases emit light when illuminated. In contrast to phosphorescence, the term fluorescence applies to substances that have no afterglow, i.e. the fluorescence ceases directly upon irradiation of the stimulator. This means that the electrons of the atoms or molecules of the fluorescent substance excited by absorbing the energy of the incident light actually spontaneously return to their basic state while emitting characteristic light. According to Stokes' Law of Fluorescence, the wavelength of light emitted by fluorescence is longer than that of absorbed light:
υe≤υa
Where
υ e = frequency of emitted radiation,
υ a = frequency of absorbed radiation.
Fluorescence is light emitted by substances that absorb light or other electromagnetic radiation. It is a form of luminescence. In most cases, the emitted light has a longer wavelength and therefore lower photon energy than the absorbed radiation. A clear example of fluorescence occurs when the absorbed radiation is in the ultraviolet region of the spectrum (invisible to the human eye) and the emitted light is in the visible region; this gives the fluorescent substance a unique color that can only be seen when exposed to ultraviolet light arrive. When the radiation source is stopped, fluorescent materials stop emitting light almost immediately, unlike phosphorescent materials, which continue to emit light after a period of time.