Demonstrations 19
Limelight
The ‘Limelight’ is the first of nine demonstrations of thermoluminescence and chemiluminescence. The experiment is based on the fact that calcium oxide luminesces at a high temperature and this is shown by the lime-light [1]. Where high temperatures are involved this emission is in addition to incandescence (black body radiation) and is in the short wavelength (blue) part of the visible spectrum. When no relevant chemical change takes place as here with calcium oxide or quicklime (CaO) and the next experiment with magnesium oxide (MgO) and finally phosphorous pentoxide (P4O10) we define it as thermoluminescence. When chemical reorientation is involved we call it chemiluminescence as in experiments 22 to 27.
Certain metal oxides emit more short wavelength light when heated than would be expected if the emission were due to incandescence alone. This was first found during the 1820’s when a young fellow by the name of Goldsworthy Gurney (later Sir Goldsworthy Gurney) played the flame of his oxy-hydrogen burner, that he had invented (MR. GURNEY’s BLOW PIPE), on various substances including a lump of lime (calcium oxide). He found that the lime gave off a brilliant white light. This near ‘point source’ of light was found to be ideal, in conjunction with lenses, to make optical systems that were soon put to use in lighthouses and in the theatre (the first spot lights), hence, to be ‘in the lime-light’. The hydrogen and oxygen gasses were stored separately in bags or bladders that were compressed with weights in order to provide sufficient pressure to feed the burners [2]. Visiting the theatre must have been rather hazardous in those days!
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Thomas Drummond made use of the long distance visibility of limelight during the Irish Survey, and is mistakenly credited with its invention; indeed, it is sometimes called Drummond light.[3, 4].
Drummond said that the limelight is of a such dazzling whiteness that it is plainly visible sixty-eight miles away, and it is reported that a sharp shadow was cast at a distance of thirteen miles. If zirconia be used in place of lime, the zircon light is obtained. According to T. Drummond the light given by zirconia is less powerful than that of lime; and that by magnesia is only half as intense. C.H. Pfaff [5] says that if the light of a wax candle be unity, the light emitted by a cylinder of lime one-fifth the diameter of the flame of the candle is 153 when heated by the oxy-hydrogen flame; 76 by the ether-oxygen flame; 69 by the alcohol-oxygen flame; and 19 by the oxygen-coal gas flame.
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Fig. 1 shows the arrangement used for producing lime-light in the middle of 19-th century. A mixture of oxygen and hydrogen was stored in the bladder from which it was forced under pressure. An alternative design involved using separate gas-bags or gasometers for storing oxygen and hydrogen and the gases were mixed in a burner, either Daniell’s jet (which we use in our demonstration) or the jet shown in Fig. 2. In the latter oxygen and hydrogen are mixed in the upper part of the jet, which is provided with a safety tube filled with circular pieces of wire gauze.
Apart from thermoluminescence calcium oxide exhibits also so-called flame luminescence. When the oxygen of an oxy-hydrogen blast lamp used in the production of the lime-light is turned off and the hydrogen flame is allowed to play over the surface of the slowly cooling lime cylinder, a greenish glow may sometimes be seen in the regions reached by the flame. This furtive glow which occurs at temperatures corresponding to a very low red heat, or below, must have been familiar to many of the early users of the projection lantern. Nichols and Wilber [6-8] found that calcium oxide exhibits a flame luminescence in the zone of hydrogen flame between oxidation and reduction; the phenomenon does not occur with the flame of alcohol, ether, sulphur or carbon disulphide. A full supply of oxygen in the atmosphere surrounding the flame is essential. Luminescence is produced by actual contact of certain zones of a hydrogen flame with the material to be excited. Rapid oxidation and reduction appear to be essential. The effect is not produced by heating in air or hydrogen outside the flame; and the effect is not modified by strong electrostatic fields, thus excluding ionisation. The spectrum is a characteristic luminescence spectrum and not a temperature spectrum. The afterglow, which is very brief, is of the type of vanishing phosphorescence. Barium and strontium oxides give a dim flame luminescence.
Preparation. No specific preparation is required. Any suitable-size piece of limestone will be adequate for this demonstration.
Demonstration. Instead of using a lump of calcium oxide which is fairly friable MFH uses a lump of limestone because it is more robust. Around the spot heated by the flame the calcium carbonate decomposes to calcium oxide or quicklime (CaO) and carbon dioxide, according to the decomposition reaction below:
CaCO3 === heat ===> CaO + CO2 (17.1)
This is the usual industrial preparation of calcium oxide e.g. in the production of cement. Old limekilns are to be found in many parts of the country, some of considerable antiquity. The small area of lime is heated to some 2000K by the flame but, in addition to the incandescent light produced, additional blue light is emitted due to thermoluminescence. The combined emission appears as an intense white light.
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The lime light was widely used in the 19-th and early 20-th centuries. This is how one of uses was described in an early book [9]. “With respect to the application of the light produced from a jet of the mixed gases [oxygen and hydrogen] thrown upon a ball of lime, it may be stated that for many years the dissolving view lanterns and other optical effects have been produced with the assistance of this light; and more lately Major Fitzmaurice has condensed the mixed gases in the old-fashioned oil gas receivers, and projected them on a ball of lime; and it was this light thrown from many similar arrangements that illuminated the British men-of-war when Napoleon III left her Majesty’s yacht at night in the docks at Cherbourg” (see Figure).
After many attempts to improve the efficiency of gas lighting a giant step was made in 1887 when the Austrian physicist Carl Auer von Welsbach made the first successful gas mantle. Most experimenters used platinum gauze coated with metal oxides including magnesium oxide (magnesia). See exp. 20. Welsbach used a fabric tube that had been soaked in a solution of soluble salts of thorium and cerium and then dried. The fabric was arranged around the burner, and when first lit it burned away leaving a delicate open structure of thorium and cerium oxides. It was so frail that it could no longer be handled. Gas mantles have changed little over the past 113 years; about 1% cerium is still used as the main thermoluminescent material but thorium, which is a little radioactive, has been replaced usually with zirconium. Today gas mantles are still produced in large quantities and used mainly in third word countries. Where electricity is ubiquitous they are used for leisure activities such as camping, and butane or propane gas is the preferred fuel. New mantles are still supplied as small cotton bags that have been soaked in the nitrates of zirconium and cerium.
Although not usually heated above, what in incandescent terms, would be a yellow heat (say 1000 oC), the mantel emits an intense white light equivalent to a ‘black body temperature’ of more than 4000 oC. In photography this ‘black body’ (incandescent equivalent) emission is known as ‘colour temperature’. The colour temperature of a light source is closely related to the ratio of the intensities of the blue and red light. Daylight type colour film has a colour temperature rating of around 5500K,† which means that it will give the correct colour rendering when exposed to light that has the same spectral quality as light that would be emitted incandescently from a ‘black body’ at that temperature. The surface of the sun is around 6000Kand since some of the blue light is scattered by our atmosphere (that’s why the sky is blue) the light reaching ground level will have a slightly lower blue to red ratio and so lower colour temperature. See also experiment 22.
Normally an electric incandescent lamp filament runs at between 2700K and 2900K. Attempts have been made to coat tungsten filaments with metallic oxides such as ThO2 so that the lamp filament could be run at a lower temperature, thus saving energy, but yet give a brighter and whiter light. Unfortunately it has been found that these metals infiltrate into the tungsten and interfere with the crystal structure leading to failure. However, where the filament runs at comparatively low temperature as in the ‘heater’ of cathode ray tubes and fluorescent lamp bulbs, these coating are common and act as electron emitters. The oxides of thorium, barium, beryllium, and other, industrially secret, recipes are used.
Curiously the oxy-hydrogen flame will burn perfectly well underwater and, surprisingly, even under liquid nitrogen! In the manufacture of quarts-halogen bulbs it in necessary to have the filler gas, usually argon (bp –186 oC / 87K) or xenon (bp –108 oC / 165K) at relatively high pressure ca. 3 bar. However it is not possible to seal the lamp envelope if the pressure inside is greater than the pressure outside. In order to melt seal the quarts, the envelope is immersed in liquid nitrogen (bp –196 oC / 77K) and the gas is condensed into the lamp ‘bulb’ from a fixed volume in another vessel. The quartz is sealed, under the liquid nitrogen, using two oxygen/hydrogen flames. The operating pressure of this type of bulb is 10 to 15 bar. For some high voltage lamps N2 is added to the noble gas to prevent arcing.
Safety. The hydrogen flame is not very luminous and is hard to see. One should not look directly at the light source for too long.
References.
1. J.W. Mellor, A Comprehensive Treatise on Inorganic and Theoretical Chemistry, vol. 3, new impr., London, Longman, Green and Co., 1956, p. 662.
2. Research by Screenhouse Productions, Leeds, for BBC2 Local Heroes 1997 (http://www.bbc.co.uk/education/archive/local_heroes97/bioggurney.shtml).
3. J.H. Pepper, The Boy’s Playbook of Science, London, Routledge, Warne & Routledge, 1860, p. 124.
4. T. Drummond, Edin. J. Sci., 1826, 5, 319.
5. C.H. Pfaff, Pogg. Ann., 1837, 40, 547.
6. E.L. Nichols and D.T. Wilber, “Flame excitation of luminescence”, Phys. Rev., Ser. 2, 1921, 17, 269.
7. E.L. Nichols and D.T. Wilber, “Flame excitation of luminescence”, Phys. Rev., Ser. 2, 1921, 17, 269, 453.
8. E.L. Nichols and D.T. Wilber, “The luminescence of certain oxides sublimed in the electric arc”, Phys. Rev., Ser. 2, 1921,17, 707.
9. J.H. Pepper, The Boy’s Playbook of Science, London, Routledge, Warne & Routledge, 1860, p. 127.