Fluorescence

Fluorescence is the process by which a substance absorbs electromagnetic radiation (visible or invisible light) from another source, then re-emits the radiation with a wavelength that is longer than the wavelength of the illuminating radiation. It can be observed in gases at low pressure and in certain liquids and solids, such as the ruby gemstone. Fluorescence is the principle that is the basis of the common fluorescent lamp used for lighting; it is also a useful laboratory diagnostic tool.