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33. Luminescence and its kinds.

Luminescence is the generation of light without heat. There are two principal varieties of luminescence, fluorescence and phosphorescence, distinguished by the delay in reaction to external electromagnetic radiation. The ancients observed phosphorescence in the form of a glow emitted by the oceans at night, and confused this phenomenon with the burning of the chemical phosphor, but, in fact, phosphorescence has nothing at all to do with burning. Likewise, fluorescence, as applied today in fluorescent lighting, involves no heat—thus creating a form of lighting more efficient than that which comes from incandescent bulbs.

Fluorescence: Luminescence that stops within 10 nanoseconds after an energy source has been removed.

Incandescence: Light created by heating. Incandescence is not a luminescent process.

Phosphor: A material that absorbs energy over some period of time, then gives off light for a longer period.

Phosphorescence: Luminescence that continues for more than 10 nanoseconds after an energy source has been removed.

Chemiluminescence, the emission of light as a result of a chemical reaction

Bioluminescence, a result of biochemical reactions in a living organism

Electrochemiluminescence, a result of an electrochemical reaction

Lyoluminescence, a result of dissolving a solid (usually heavily irradiated) in a liquid solvent

Crystalloluminescence, produced during crystallization

Electroluminescence, a result of an electric current passed through a substance

Cathodoluminescence, a result of a luminescent material being struck by electrons

Mechanoluminescence, a result of a mechanical action on a solid

Triboluminescence, generated when bonds in a material are broken when that material is scratched, crushed, or rubbed

Fractoluminescence, generated when bonds in certain crystals are broken by fractures

Piezoluminescence, produced by the action of pressure on certain solids

Sonoluminescence, a result of imploding bubbles in a liquid when excited by sound

Photoluminescence, a result of absorption of photons

Radioluminescence, a result of bombardment by ionizing radiation

Thermoluminescence, the re-emission of absorbed energy when a substance is heated

Cryoluminescence, the emission of light when an object is cooled (an example of this is wulfenite)

34. Stimulated radiation. Laser.

Stimulated radiation, in laser action, the release of energy from an excited atom by artificial means. According to Albert Einstein, when more atoms occupy a higher energy state than a lower one under normal temperature equilibrium (see population inversion), it is possible to force atoms to return to an unexcited state by stimulating them with the same energy as would be emitted naturally. In stimulated radiation the emitted light wave will be coherent (i.e., in phase; see coherence) with the incoming wave. In laser action the stimulating emission triggers a chain reaction in which the radiation from one atom stimulates another in succession until all the excited atoms in the system have returned to normalcy. In doing so, coherent monochromatic light (light of a single wavelength) is emitted.

Stimulated emission can be modelled mathematically by considering an atom that may be in one of two electronic energy states, a lower level state and an excited state, with energies E1 and E2 respectively.

If the atom is in the excited state, it may decay into the lower state by the process of spontaneous emission, releasing the difference in energies between the two states as a photon. The photon will have frequency ν and energy hν, given by:E2-E1=hv0 where h is Planck's constant

A laser is a device that emits light through a process of optical amplification based on the stimulated emission of electromagnetic radiation. The term "laser" originated as an acronym for "light amplification by stimulated emission of radiation".The first laser was built in 1960 by Theodore H. Maiman at Hughes Research Laboratories, based on theoretical work by Charles Hard Townes and Arthur Leonard Schawlow.

Laser is a Light Amplification by Stimulated Emission Radiation. A laser radiation source is provided with an array of individual slave laser diodes in a predetermined surface area. Laser has certain unique properties: high monochromaticity, coherence and directionality.

  • Gas Lasers- the use of these lasers ranges from bar code scanning to laser eye surgery.

  • Chemical Lasers- mainly used as weaponry by the military.

  • Dye Lasers- used to remove birth marks

  • Metal-vapor lasers- used for printing, dermatology, and high speed photography.

  • Solid-state lasers- used for tattoo removal, spectroscopy, and many other uses.

  • Semiconductor lasers- used for telecommunications, holography, printing, welding etc.

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