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Fluorescent minerals

Fluorescence is the emission of electromagnetic radiation light by a substance that has absorbed radiation of a different wavelength. In most cases, absorption of light of a certain wavelength induces the emission of light with a larger wavelength (and lower energy). However, under conditions in which intense radiation is being absorbed, it is possible for one electron to absorb two photons (multiple photon absorption), which can lead to the emission of radiation having a smaller wavelength than the excitation source. The energy difference between the absorbed and emitted photons is due to thermal losses. Dissipation of vibrational energy occurs on a much greater time scale than fluorescent emission. The most striking examples of this phenomenon occur when the absorbed photon is in the ultraviolet region of the spectrum, and is thus invisible, and the emitted light is in the visible region. Practical applications of this effect are found in mineralogy, gemology, chemical sensors, fluorescent labelling, dyes, biological detectors etc. Newer applications of fluorescent compounds are being explored daily.

The term 'fluorescence' was coined by George Gabriel Stokes in his 1852 paper;[1] the name was suggested "to denote the general appearance of a solution of sulphate of quinine and similar media". (Phil. Trans. R. Soc. Lond. 1853 143, 385-396, quote from page 387). The name itself was derived from the mineral fluorite (calcium difluoride), some examples of which contain traces of divalent europium, which serves as the fluorescent activator to provide a blue fluorescent emission. The fluorite which provoked the observation originally, and which remains one of the most outstanding examples of the phenomenon, originated from the Weardale region, of northern England.





Fluorescence occurs when an orbital electron of a molecule, atom or nanostructure relaxes to its ground state after being excited to a higher quantum state by some type of energy:

Excitation:  S_0 + h \nu_{ex} \to S_1

Fluorescence (emission):  S_1 \to S_0 + h \nu_{em} + heat

here hν is a generic term for photon energy with h = Planck's constant and ν = frequency of light. (The specific frequencies of exciting and emitted light are dependent on the particular system.)

State S0 is called the ground state of the fluorophore (fluorescent molecule) and S1 is its first (electronically) excited state.

A molecule in its excited state, S1, can relax by various competing pathways. It can undergo 'non-radiative relaxation' in which the excitation energy is dissipated as heat (vibrations) to the solvent. Excited organic molecules can also relax via conversion to a triplet state which may subsequently relax via phosphorescence or by a secondary non-radiative relaxation step.

Relaxation of an S1 state can also occur through interaction with a second molecule through fluorescence quenching. Molecular oxygen (O2) is an extremely efficient quencher of fluorescence just because of its unusual triplet ground state.

Molecules that are excited through light absorption or via a different process (e.g. as the product of a reaction) can transfer energy to a second 'sensitized' molecule, which is converted to its excited state and can then fluoresce. This process is used in lightsticks to produce different colors.

Quantum yield

The fluorescence quantum yield gives the efficiency of the fluorescence process. It is defined as the ratio of the number of photons emitted to the number of photons absorbed.

 \Phi = \frac {\rm \#\ photons \ emitted} {\rm \#\ photons \ absorbed}

The maximum fluorescence quantum yield is 1.0 (100%); every photon absorbed results in a photon emitted. Compounds with quantum yields of 0.10 are still considered quite fluorescent. Another way to define the quantum yield of fluorescence, is by the rates excited state decay:

 \frac{ { k}_{ f} }{ \sum_{i}{ k}_{i } }

where kf is the rate of spontaneous emission of radiation and


is the sum of all rates of excited state decay. Other rates of excited state decay are caused by mechanisms other than photon emission and are therefore often called "non-radiative rates", which can include: dynamic collisional quenching, near-field dipole-dipole interaction (or resonance energy transfer), internal conversion and intersystem crossing. Thus, if the rate of any pathway changes, this will affect both the excited state lifetime and the fluorescence quantum yield.

Fluorescence quantum yield are measured by comparison to a standard with known quantology; the quinine salt, quinine sulfate, in a sulfuric acid solution is a common fluorescence standard.


The fluorescence lifetime refers to the average time the molecule stays in its excited state before emitting a photon. Fluorescence typically follows first-order kinetics:

 \left[S 1 \right] = \left[S 1 \right]_0 e^{-\Gamma t}

where \left[S 1 \right] is the concentration of excited state molecules at time t, \left[S 1 \right]_0 is the initial concentration and Γ is the decay rate or the inverse of the fluorescence lifetime. This is an instance of exponential decay. Various radiative and non-radiative processes can de-populate the excited state. In such case the total decay rate is the sum over all rates:

Γtot = Γrad + Γnrad

where Γtot is the total decay rate, Γrad the radiative decay rate and Γnrad the non-radiative decay rate. It is similar to a first-order chemical reaction in which the first-order rate constant is the sum of all of the rates (a parallel kinetic model). If the rate of spontaneous emission, or any of the other rates are fast, the lifetime is short. For commonly used fluorescent compounds typical excited state decay times for fluorescent compounds that emit photons with energies from the UV to near infrared are within the range of 0.5 to 20 nanoseconds. The fluorescence lifetime is an important parameter for practical applications of fluorescence such as fluorescence resonance energy transfer.


There are several rules that deal with fluorescence. The Kasha–Vavilov rule dictates that the quantum yield of luminescence is independent of the wavelength of exciting radiation.

This is not always true and is violated severely in many simple molecules. A somewhat more reliable statement, although still with exceptions, would be that the fluorescence spectrum shows very little dependence on the wavelength of exciting radiation.

The Jablonski diagram describes most of the relaxation mechanism for excited state molecules.


There are many natural and synthetic compounds that exhibit fluorescence, and they have a number of applications. Some deep-sea animals, such as the Greeneye, use fluorescence.


Fluorescent paint and plastic lit by UV tubes.

The common fluorescent tube relies on fluorescence. Inside the glass tube is a partial vacuum and a small amount of mercury. An electric discharge in the tube causes the mercury atoms to emit ultraviolet light. The tube is lined with a coating of a fluorescent material, called the phosphor, which absorbs the ultraviolet and re-emits visible light. Fluorescent lighting is very energy-efficient compared to incandescent technology, but the spectra produced may cause certain colours to appear unnatural. A compact fluorescent lamp can replace incandescent lighting. The mercury vapor emission spectrum is dominated by a short-wave UV line at 254 nm (which provides most of the energy to the phosphors), accompanied by visible light emission at 436 nm (blue), 546 nm (green) and 579 nm (yellow-orange). These three lines can be observed superimposed on the white continuum using a hand spectroscope, for light emitted by the usual white fluorescent tubes. These same visible lines, accompanied by the emission lines of trivalent europium and trivalent terbium, and further accompanied by the emission continuum of divalent europium in the blue region, comprise the more discontinuous light emission of the modern trichromatic phosphor systems used in helical light bulbs.

Fluorescent lights were first available to the public at the 1939 World's Fair, then in the mid 1990s, white light-emitting diodes (LEDs) became available, which work through a similar process. No significant wide-scale manufacturing and innovation of light technology emerged since fluorescents, in 1939, until the (LEDs). Typically, the actual light-emitting semiconductor produces light in the blue part of the spectrum, which strikes a phosphor compound deposited on the chip; the phosphor fluoresces from the green to red part of the spectrum. The combination of the blue light that goes through the phosphor and the light emitted by the phosphor produce a net emission of white light.

Glow sticks sometimes utilize fluorescent materials to absorb light from the chemiluminescent reaction and emit light of a different color.

Analytical chemistry

Fluorescence in several wavelengths can be detected by an array detector, to detect compounds from HPLC flow. Also, TLC plates can be visualized if the compounds or a coloring reagent is fluorescent. Fluorescence is most effective when there is a larger ratio of atoms at lower energy levels in a Boltzmann distribution. There is then a higher probability of lower energy atoms being excited and releasing photons, making analysis more efficient.

Fingerprints can be visualized with fluorescent compounds such as ninhydrin.


Usually the setup of a Fluorescence assay involves a Light source, which may emit an array different wavelengths of light. Generally, only one wavelegth is required for proper analysis, so in order to selectively filter the light, it is passed through an excitation monochromator, and then that chosen wavelength is passed through the sample cell. After absorption and re-emission of the energy, many wavelengths may various energies due to Stokes shift and various electron transitions. Therefore, the wavelegths are passed through an Emission monochromator, and selectively observed by a detector [2]

Biochemistry and medicine

Endothelial cells under the microscope with three separate channels marking specific cellular components

Fluorescence in the life sciences is used generally as a non-destructive way of tracking or analysis biological molecules by means of the fluorescent emission at a specific frequency where there is no background from the excitation light, as relatively few cellular components are naturally fluorescent (called intrinsic or autofluorescence). In fact, a protein or other component can be "labelled" with a extrinsic fluorophore, a fluorescent dye which can be a small molecule, protein or quantum dot, finding a large use in many biological applications.[3] [4]
The quantification of a dye is done with a Spectrofluorometer and finds additional applications in:

  • when scanning the fluorescent intensity across a plane one has Fluorescence microscopy of tissues, cells or subcellular structures which is accomplished by labeling an antibody with a fluorophore and allowing the antibody to find its target antigen within the sample. Labeling multiple antibodies with different fluorophores allows visualization of multiple targets within a single image (multiple channels). DNA microarrays are a variant of this.
  • Automated sequencing of DNA by the chain termination method; each of four different chain terminating bases has its own specific fluorescent tag. As the labeled DNA molecules are separated, the fluorescent label is excited by a UV source, and the identity of the base terminating the molecule is identified by the wavelength of the emitted light.

. Ethidium bromide fluoresces orange when intercalating DNA and when exposed to UV light.

  • FACS (fluorescent-activated cell sorting)
  • DNA detection: the compound ethidium bromide, when free to change its conformation in solution, has very little fluorescence. Ethidium bromide's fluorescence is greatly enhanced when it binds to DNA, so this compound is very useful in visualising the location of DNA fragments in agarose gel electrophoresis. Ethidium bromide may be carcinogenic - an arguably safer alternative is the dye SYBR Green.
  • Immunology: An antibody has a fluorescent chemical group attached, and the sites (e.g., on a microscopic specimen) where the antibody has bound can be seen, and even quantified, by the fluorescence.

Additionally Fluorescence resonance energy transfer used to study protein interactions, detect specific nucleic acid sequences and used as biosensors, while fluorescent lifetime can give an additional layer of information.

Gemology, mineralogy, geology, and forensics

Gemstones, minerals, fibers, and many other materials which may be encountered in forensics or with a relationship to various collectibles may have a distinctive fluorescence or may fluoresce differently under short-wave ultraviolet, long-wave ultra violet, or X-rays.

Many types of calcite and amber will fluoresce under shortwave UV. Rubies, emeralds, and the Hope Diamond exhibit red fluorescence under short-wave UV light; diamonds also emit light under X ray radiation.

Fluorescence in minerals is caused by a wide range of activators. In some cases, the concentration of the activator must be restricted to below a certain level, to prevent quenching of the fluorescent emission. Furthermore, certain impurities such as iron or copper need to be absent, to prevent quenching of possible fluorescence. Divalent manganese, in concentrations of up to several percent, is responsible for the red or orange fluorescence of calcite, the green fluorescence of willemite, the yellow fluorescence of esperite, and the orange fluorescence of wollastonite and clinohedrite. Hexavalent uranium, in the form of the uranyl cation, fluoresces at all concentrations in a yellow green, and is the cause of fluorescence of minerals such as autunite or andersonite, and, at low concentration, is the cause of the fluorescence of such materials as some samples of hyalite opal. Trivalent chromium at low concentration is the source of the red fluorescence of ruby corundum. Divalent europium is the source of the blue fluorescence, when seen in the mineral fluorite. Trivalent lanthanoids such as terbium and dysprosium are the principal activators of the creamy yellow fluorescence exhibited by the yttrofluorite variety of the mineral fluorite, and contribute to the orange fluorescence of zircon. Powellite (calcium molybdate) and scheelite (calcium tungstate) fluoresce intrinsically in yellow and blue, respectively. When present together in solid solution, energy is transferred from the higher energy tungsten to the lower energy molybdenum, such that fairly low levels of molybdenum are sufficient to cause a yellow emission for scheelite, instead of blue. Low-iron sphalerite (zinc sulfide), fluoresces and phosphoresces in a range of colors, influenced by the presence of various trace impurities.

Crude oil (petroleum) fluoresces in a range of colors, from dull brown for heavy oils and tars through to bright yellowish and bluish white for very light oils and condensates. This phenomenon is used in oil exploration drilling to identify very small amounts of oil in drill cuttings and core samples.

Organic liquids

Organic liquids such as mixtures of anthracene in benzene, toluene, or stilbene in the same solvents, fluoresce with ultraviolet or gamma ray irradiation. The decay times of this fluorescence is of the order of nanoseconds since the duration of the light depends on the lifetime of the excited states of the fluorescent material, in this case anthracene or stilbene.

See also

Further reading

  • Lakowicz, J.R. 2006. Principles of Fluorescence Spectroscopy, Third Edition, Plenum Press, New York. ISBN 0-387-31278-1.
  • Valeur, B. 2001. Molecular Fluorescence: Principles and Applications, Wiley-VCH. ISBN 352729919X .
  • Guilbault, G.G. 1990. Practical Fluorescence, Second Edition, Marcel Dekker, Inc., New York. ISBN 0-8247-8350-6.


  1. ^ Stokes, G. G. (1852). "On the Change of Refrangibility of Light". Philosophical Transactions of the Royal Society of London 142: 463–562. 
  2. ^ Harris, D. C. et al, Exploring Chemical Analysis 4th ed., New York, NY (c)2009 by W.H. Freeman and Company
  3. ^ Lakowicz, J.R., Principles of fluorescence spectroscopy. 3rd ed. 2006, New York: Springer. xxvi, 954 p.
  4. ^ Invitrogen.com
  5. ^ X. Chen, Z. Mutasim, J. Price, J. P. Feist, A. L. Heyes and S. Seefeldt (2005), "Industrial sensor TBCs: Studies on temperature detection and durability", International Journal of Applied Ceramic Technology, Vol. 2, No. 5, pp. 414-421.
  6. ^ A. L. Heyes, S. Seefeldt, J. P Feist (2005), ‘Two-colour thermometry for surface temperature measurement’, Optics and Laser Technology, 38, pp.257-265.
  7. ^ R. J. L. Steenbakker, J. P. Feist, R. G. Wellmann, J. R. Nicholls, (2008), "Sensor TBC's: Remote n-itu condition monitoring of EB-PVD coatings at elevated temperatures E", GT2008-51192, Proceedings of ASME Turbo Expo 2008: Power for Land, Sea and Air,June 9–13, 2008, Berlin, Germany.
  8. ^ J. P. Feist, A. L. Heyes and J. R. Nicholls (2001), "Phosphor thermometry in an electron beam physical vapour deposition produced thermal barrier coating doped with dysprosium", Proceedings of Institution of Mechanical Engineers, Vol. 215 Part G, pp. 333-340.

External links

1911 encyclopedia

Up to date as of January 14, 2010

From LoveToKnow 1911

FLUORESCENCE. In a paper read before the Royal Society of Edinburgh in 1833, Sir David Brewster described a remarkable phenomenon he had discovered to which he gave the name of "internal dispersion." On admitting a beam of sunlight, condensed by a lens, into a solution of chlorophyll, the green colouring matter of leaves (see fig. I), he was surprised to find that the path of the rays within the fluid was marked by a bright light of a blood-red colour, strangely contrasting with the beautiful green of the fluid when seen in moderate thickness. Brewster afterwards observed the same phenomenon in various vegetable solutions and essential oils, and in some solids, amongst which was fluor-spar. He believed this effect to be due to coloured particles held in suspension. A few years later, Sir John Herschel independently discovered that if a solution of quinine sulphate, which, viewed by transmitted light, appears colourless and transparent like water, were illuminated by a beam of ordinary daylight, a peculiar blue colour was seen in a thin stratum of the fluid adjacent to the surface by which the light entered. The blue light was unpolarized and passed freely through many inches of the fluid. The incident beam, after having passed through the stratum from which the blue light came, was not sensibly enfeebled or coloured, but yet it had lost the power of FIG. I.

producing the characteristic blue colour when admitted into a second solution of quinine sulphate. A beam of light modified in this mysterious manner was called by Herschel "epipolized." Brewster showed that epipolic was merely a particular case of internal dispersion, peculiar only in this respect, that the rays capable of dispersion were dispersed with unusual rapidity.

The investigation of this phenomenon was afterwards taken up by Sir G. G. Stokes, to whom the greater part of our present knowledge of the subject is due. Stokes's first paper "On the Change of the Refrangibility of Light" appeared in 1852. He repeated the experiments of Brewster and Herschel, and considerably extended them. These experiments soon led him to the conclusion that the effect could not be due, as Brewster had imagined, to the scattering of light by suspended particles, but that the dispersed beam actually differed in refrangibility from the light which excited it. He therefore termed it "true internal dispersion" to distinguish it from the scattering of light, which he called "false internal dispersion." As this name, however, is apt to suggest Brewster's view of the phenomenon, he afterwards abandoned it as unsatisfactory, and substituted the word "fluorescence." This term, derived from fluor-spar after the analogy of opalescence from opal, does not presuppose any theory. To examine the 'nature of the fluorescence produced by quinine, Stokes formed a pure spectrum of the sun's rays in the usual manner. A test-tube, filled with a dilute solution of quinine sulphate, was placed just outside the red end of the spectrum and then gradually moved along the spectrum to the other extremity. No fluorescence was observed as long as the tube remained in the more luminous portion, but as soon as the violet was reached, a ghost-like gleam of blue light shot right across the tube. On continuing to move the tube, the blue light at first increased in intensity and afterwards died away, but not until the tube had been moved a considerable distance into the ultra-violet part of the spectrum. When the blue gleam first appeared it extended right across the tube, but just before disappearing it was confined to a very thin stratum on the side at which the exciting rays entered. Stokes varied this experiment by placing a vessel filled with the dilute solution in a spectrum formed by a train of prisms. The appearance is illustrated diagrammatically in fig. 2. The greater part of the light passed freely as if through water, but from about half-way between the Fraunhofer lines G and H to far beyond the extreme violet, the incident rays gave rise to light of a sky-blue colour, which emanated in all directions from the portion of the fluid (represented white in fig. 2) which was under the influence of the incident rays. The anterior surface of the blue space coin FIG. 2. cided, of course, with the inner surface of the glass vessel. The posterior surface marked the distance to which the incident rays were able to penetrate before they were absorbed. This distance was at first considerable, greater than the diameter of the vessel, but decreased with great rapidity as the refrangibility of the incident light increased, so that from a little beyond the extreme violet to the end, the blue space was reduced to an excessively thin stratum. This shows that the fluid is very opaque to the ultraviolet rays. The fixed lines in the violet and invisible part of the solar spectrum were represented by dark lines, or rather planes, intersecting the blue region. Stokes found that the fluorescent light is not homogeneous, for on reducing the incident rays to a narrow band of homogeneous light, and examining the dispersed beam through a prism, he found that the blue light consisted of rays extending over a wide range of refrangibility, but not into the ultra-violet.

Another method, which Stokes found especially useful in examining different substances for fluorescence, was as follows. Two coloured media were prepared, one of which transmitted the upper portion of the spectrum and was opaque to the lower portion, while the second was opaque to the upper and trans parent to the lower part of the spectrum. These were called by Stokes "complementary absorbents." No pair could be found which were exactly complementary, of course, but the condition was approximately fulfilled by several sets of coloured glasses or solutions. One such combination consisted of a deep-blue solution of ammoniacal copper sulphate and a yellow glass coloured with silver. The two media together were almost opaque. The light of the sun being admitted through a hole in the window-shutter, a white porcelain tablet was laid on a shelf fastened in front of the hole. If the vessel containing the blue solution was placed so as to cover the hole, and the tablet was viewed through the yellow glass, scarcely any light entered the eye, but if a paper washed with some fluorescent liquid were laid on the tablet it appeared brilliantly luminous. Different pairs of complementary absorbents were required according to the colour of the fluorescent light. This experiment shows clearly that the light which passed through the first absorbent and which would have been stopped by the second gave rise in the fluorescent substance to rays of a different wave-length which were transmitted by the second absorbent. Scattered light, with which the true fluorescent light was often associated, was eliminated by this method, being stopped by the second absorbent.

Stokes also used a method, analogous to Newton's method of crossed prisms, for the purpose of analysing the fluorescent light. A spectrum was produced by means of a slit and a prism, the slit being horizontal instead of vertical. The resulting very narrow spectrum was projected on a white paper moistened with a fluorescent solution, and viewed through a second prism with its refracting edge per pendicular to that of the first prism. In addition to the sloping spectrum seen under ordinary circumstances, another spectrum due to the fluorescent light alone, made its appearance, as seen in figs. 3 and 4. In this spectrum the colours do not run from left to right, but in horizontal lines. Thus the dark lines of the solar spectrum lie across the colours. The spectra in figs. 3 and 4 were obtained by V. Pierre with an improved arrange ment of Stokes's method. It will be seen that, in the case of chlorophyll, the whole spectrum, far into the ultra-violet, gives rise to a short range of red fluorescent light, while the effective part of the exciting light in the case of aesculin (a glucoside occurring in horse-chestnut bark) begins a little above the fixed line G and the fluorescent light covers a wide range extending from orange to blue.

Besides the substances already mentioned, a large number of vegetable extracts and some inorganic bodies are strongly fluorescent. Stokes found that most organic substances show signs of fluorescence. Green fluor-spar from Alston Moor exhibits a violet, uranium glass a yellowish-green fluorescence. Tincture of turmeric gives rise to a greenish light, and the extract of seeds of Datura stramonium a pale green light. Ordinary paraffin oil fluoresces blue. Barium platinocyanide, which is much used in the fluorescent screens employed in work with the Röntgen rays, shows a brilliant green fluorescence with ordinary light. Crystals of magnesium platinocyanide possess the remarkable property of emitting a polarized fluorescent light, a. ' 'D ' 'F G H FIG. 3. - Spectrum of Chlorophyll.

H FIG. 4. - Spectrum of Aesculin.

C the colour and plane of polarization depending on the position of the crystal with respect to the incident beam, and, if polarized light is used, on the plane of polarization of the latter.

Stokes's Law

In all the substances examined by Stokes, the fluorescent light appeared to be of lower refrangibility than the light which excited it. Stokes considered it probable that this lowering of the refrangibility of the light was a general law which held for all substances. This is known as Stokes's law. It has been shown, however, by E. Lommel and others, that this law does not hold generally. Lommel distinguishes two kinds of fluorescence. The bodies which exhibit the first kind are those which possess strong absorption bands, of which only one remains appreciable after great dilution. These bodies are always strongly coloured and show anomalous dispersion and (in solids) surface colour. In such cases, the maximum of intensity in the fluorescent spectrum corresponds to the maximum of absorption. Stokes's law is not obeyed, for a fluorescent spectrum can be produced by means of homogeneous light of lower refrangibility than a great part of the fluorescent light. The second kind of fluorescence is the most common, and is exhibited by bodies which show absorption only in the upper part of the spectrum, i.e. they are usually yellow or brown or (if the absorption is in the ultra-violet) colourless. The absorption bands also are different from those of substances of the first kind, for they readily disappear on dilution. A third class of bodies is formed by those substances which exhibit both kinds of fluorescence.

Nature of Fluorescence

No complete theory of fluorescence has yet been given, though various attempts have been made to explain the phenomenon. Fluorescence is closely allied to phosphorescence, the difference consisting in the duration of the effect after the exciting cause is removed. Liquids which fluoresce only do so while the exciting light is falling on them, ceasing immediately the exciting light is cut off. In the case of solids, on the other hand, such as fluor-spar or uranium glass, the effect, though very brief, does not die away quite instantaneously, so that it is really a very brief phosphorescence. The property of phosphorescence has been generally attributed to some molecular change taking place in the bodies possessing it. That some such change takes place during fluorescence is rendered probable by the fact that the property depends upon the state of sensitive substance; some bodies, such as barium platinocyanide, fluorescing in the solid state but not in solution, while others, such as fluorescein, only fluoresce in solution. Fluorescence is always associated with absorption, but many bodies are absorbent without showing fluorescence. A satisfactory theory would have to account for these facts as well as for the production of waves of one period by those of another, and the non-homogeneous character of the fluorescent light. Quite recently W. Voigt has sought to give a theory of fluorescence depending on the theory of electrons. Briefly, this theory assumes that the electrons which constitute the molecule of the sensitive body can exist in two or more different configurations simultaneously, and that these are in dynamical equilibrium, like the molecule in a partially dissociated gas. If the electrons have different periods of vibration in the different configurations, then it would happen that the electrons whose period nearly corresponded with that of the incident light would absorb the energy of the latter, and if they then underwent a transformation into a different configuration with a different period, this absorbed energy would be given out in waves of a period corresponding to that of the new configuration.

Applications of Fluorescence

The phenomenon of fluorescence can be utilized for the purpose of illustrating the laws of reflection and refraction in lecture experiments since the path of a ray of light through a very dilute solution of a sensitive substance is rendered visible. The existence of the dark lines in the ultra-violet portion of the solar spectrum can also be demonstrated in a simple manner. In addition to the foregoing applications, Stokes made use of this property for studying the character of the ultra-violet spectrum of different sources of illumination and flames. He suggested also that the property would in some cases furnish a simple test for the presence of a small quantity of a sensitive substance in an organic mixture. Fluorescent screens are largely used in work with Röntgen rays. There appears to be some prospect of light being thrown on the question of molecular structure by experiments on the fluorescence of vapours. Some very interesting experiments in this direction have been performed by R. W. Wood on the fluorescence of sodium vapour.

REFERENCES.-Sir G. G. Stokes, Mathematical and Physical Papers, vols. iii. and iv.; Muller-Pouillet, Lehrbuch der Physik, Bd. ii. (1897); A. Wullner, Lehrbuch der Experimentalphysik, Bd. iv. (1899); A. A. Winkelmann, Handbuch der Physik, Bd. vi. (1906); R. W. Wood, Physical Optics (1905). (J. R. C.)

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