is the property of a material to emit light of a certain wavelength after it has been irradiated with light of another wavelength (usually of higher energy).

Atoms or molecules that contain one or several unpaired electrons and, in that sense, are „free“. Correspondingly, any free radical has an electron that stands alone. Many free radicals are highly reactive, meaning that they have a strong tendency to arrange in pairs and, thus, escape the instable unpaired condition. For pairing, free radicals randomly receive electrons from a suitable donor or donate electrons to an appropriate acceptor, which converts into secondary free radicals. Such chain reactions may cause biological damage.