Fluorescence

Fluorescence, pronounced floo uh REHS uhns, is a process by which a variety of substances give off light or another form of electromagnetic radiation when they absorb energy. The term fluorescence also refers to the light emitted by these substances. Many gases, liquids, and solids become fluorescent when exposed to radiation or to electrically charged particles.

Fluorescent lighting is widely used in factories, offices, and schools and in some homes. Many highway lights contain fluorescent mercury vapor. TV picture tubes and electron microscopes have fluorescent screens. Biologists stain cells and tissues with fluorescent dyes to observe biological processes. Chemists detect certain air and water pollutants by using fluorescence. It also is used to detect lead poisoning and to identify minerals.

The color of fluorescing light depends on the substance involved and on the type of energy absorbed. In most cases, the wavelengths of fluorescing light are longer than those of the absorbed radiation. However, fluorescence has been observed throughout the visible portion of the electromagnetic spectrum and also in its ultraviolet and infrared regions (see Electromagnetic Waves).

Many kinds of energy cause fluorescence. For example, electric current produces fluorescence in neon signs. Ultraviolet rays, visible light, X rays, and various other forms of radiation also cause fluorescence.

When a fluorescent substance absorbs energy, electrons in its atoms become excited--that is, their energy level increases. In some cases, the electrons remain excited for only 1 trillionth of a second. The excess energy is emitted as electromagnetic radiation. The process stops when the energy source is removed.

Fluorescence was observed by scientists as early as the mid-1500's. It was first explained in 1852 by Sir George G. Stokes, a British physicist, who named the light.

Contributor: Peter M. Rentzepis, Ph.D., Presidential Chair and Prof. of Chemistry, Univ. of California, Irvine.

Fluorescence

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