Monday, February 7, 2011

LUMINESCENCE

           Luminescence is the general name gicen to the emission of light from a substance, adn ehich is producesd by some reaction other than burning. Chemical luminescence is consequent upon certain chemical reactions; an example is the glowing osf phosphorus in solution in olive oil. Thermoluminescence is excited by heating or cooling ; green fluorspar fragments thrown upon a hot surface shine brightly, whilst quinine gives off light on cooling from a high temperature.
          Frictional luminescence is shown by cane sugar adn by a few specimens of diamond   Crystallisation luminescence is shown by a 20% solution of arsenious oxide in boiling conc, HCI, which emits a flash as every crystal is formed . 
          From a textile point of view , fluorescence and phosphorescence are of most importance. Phosphorescent substances after having been exposed to light glow, and continue to glow, for some time after the light sourxe has been cut off; examples are luminous paint ( calcium sulphide ) and , at very low temperatures, ordinary paper . Fluorescence describes the emission of light of one wave-length when exposed to light of a different ( and usually shorter ) wave- length ; the fluorescence ceases immediately the light source is cut off , thus distingushing it from phosphorescence; an example is the yellowish green fluorescence of Rhodamine concurrently with the absorption of green light of a certain wave-lingth.

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