Luminescence

“ There is thus ample evidence that growth and respiration are properties quite distinct and separable from luminescence. Indeed, respiration increases continuously up to a relatively high maximum whereas luminescence falls off rapidly above a relatively low optimum. ”

“ If the radiant energy is light (visible or ultra-violet) we speak of photoluminescence, if cathode rays we have cathodoluminescence, if anode rays, anodoluminescence, and if X-rays (Röntgen rays) we have radioluminescence. Inasmuch as the α, β, and γ rays of radium correspond to the anode, cathode, and X-rays, respectively, radium radiation also produces luminescence in many kinds of material. If the material gives off the light only during the time it is radiated we speak of fluorescence; if the light persists we speak of phosphorescence. ”

“ V=violet and R=red end of visible spectrum. (Courtesy Macmillan Co.)
A body which emits light because of its (high) temperature is said to be incandescent and we speak of temperature radiation. We know, however, of many cases where substances give off light at temperatures much below 525° C. They do not follow the Stefan-Boltzmann law. The light emission is stimulated by some other means than heat. Such bodies we speak of as luminescent, and in this category belong all luminous animals. The distinction between light and luminescence was first pointed out by Wiedemann (1888) . ”

“ Then suddenly, not a hundred yards from us, gushed out a flood of soft radiance, opalescent, filled with pearly glimmerings and rosy shadows of light.
It was as though a door had opened into some world of luminescence. From it the lambent torrent poured; billowed down upon us. In its wake came music—if music the mighty harmonies, the sonorous chords, the crystalline themes and the linked chaplet of notes that were like spiralings of tiny golden star bells could be named. ”

“ LIGHTING. Artificial light is generally produced by raising some body to a high temperature. If the temperature of a solid body be greater than that of surrounding bodies it parts with some of its energy in the form of radiation. Whilst the temperature is low these radiations are not of a kind to which the eye is sensitive; they are exclusively radiations less refrangible and of greater wave-length than red light, and may be called infra-red. As the temperature is increased the infra-red radiations increase, but presently there are added radiations which the eye perceives as red light. ”

“ Each globule of moisture floating in the air becomes a reflector of light, and by myriads of reflections and counter reflections the light (which on a clear night is concentrated) is diffused over a large area, producing an illumination which for practical purposes is far superior to that produced on a clear night. When the latter condition prevails the rays of light are so intense on objects immediately surrounding the lamps that one is blinded; so that the places which are in shadow seem darker than they would be if there were no light at all. ”

“ Two bright lights may be made to produce darkness.—If two pencils of light radiate from two spots very close to each other in such a manner that they cross each other at a given point, any object placed at that line of interference will be illuminated with the sum of the two luminous pencils. If we suppose those rays to move in waves, and the elevation of the wave to represent the maximum of luminous effect, then the two waves meeting, when they are both at the height of their undulation, will necessarily produce a spot of greater intensity. ”

“ Heat and other conditions of the medium probably have a share in these operations of color. Still, the peculiar and powerful effect of luminous radiation as a part of them is beyond dispute.
The whole system of organic functions shares in the benefits of light. Darkness seems to favor the preponderance of the lymphatic system, a susceptibility to catarrh in the mucous membranes, flaccidity of the soft parts, swellings and distortions of the bony system, etc. Miners and workmen employed in ill-lighted shops are exposed to all these causes of physiological suffering. ”

“ Visible radiation × visual sensibility is spoken of as luminosity; (3) by the amount of light (expressed in candles) produced in relation to a given expenditure of energy or in relation to the cost of the energy expended. Thus, of the radiation emitted from an incandescent electric lamp only a small per cent. is light, the rest being heat and actinic rays. It is therefore very far from being 100 per cent. efficient. If there were no infra-red or ultra-violet in the radiation from an incandescent lamp its efficiency would be 100 per cent. if we disregarded visual sensibility. ”

“ On the other hand, some other objects, such as a lighted lamp, a burning coal, or a red hot iron, would be seen if no outside light were present. Such bodies are said to be luminous. Most luminous bodies are hot and become non-luminous on cooling. There are, however, some bodies that are luminous at ordinary room temperatures, as the firefly and some phosphorescent paints. When light emitted by a luminous body strikes an object, a portion of it is always reflected. ”

“ Consequently two kinds of light were given off by this star; one forming a series of bright lines indicative of luminous gas, the other consisting of a continuous spectrum, crossed by dark lines, showing the existence of a solid body in a state of incandescence, surrounded by a photosphere of luminous vapours. Two of the bright lines undoubtedly showed the presence of hydrogen in a state of illumination, the great brightness of the lines indicating that the burning gas was hotter than the photosphere. ”

“ It is defined as a substance hot to the feel. [Heat, then, and light are identified as one substance.] [3]
"It has the qualities of earth except smell, taste, and gravity. It is eternal, as atoms; not so, as aggregates.
"Organic luminous bodies are beings abiding in the solar realm. The visual ray, which is the organ of sight, is lucid. Unorganic light is reckoned fourfold: earthy, celestial, alvine and mineral. Another distinction concerns sight and feel; as light or heat may be either latent or manifest, in respect of both sight and feel, or differently in regard to either. ”

“ Responses to luminous stimulation occur in all cases equally well, whether the light employed be direct sunlight, diffused daylight, polarized light, or any of the luminous rays of the spectrum employed separately. On the other hand, neither the non-luminous rays beyond the red, nor those beyond the violet, appear to exert the smallest degree of stimulating effect. Hence, in all respects, the rudimentary eye of Sarsia appears to be affected by the same qualities of light as are our own. ”

“ These nerve vibrations are sent over the optic nerve to the brain, where they take on the shape, size and colors of the Sun.
Reflection of Light.—The flame of a match, or a candle, an oil or a gas lamp, or Sun, comet or meteor, produces its own light, and for this reason these bodies are called self-luminous—that is, they are themselves the source of light.
All other objects which do not produce light, such as an apple, a stone, the Earth and other planets and moons are called non-luminous bodies. ”

“ In diffused lighting, the lamp is covered, as by frosting, so that the rays of light are broken up and so scattered that no direct ray shines into the eyes, and there is no bright spot of light in the room. ”

“ When the sun failed to shine, or was not convenient, I practiced looking at a large incandescent electric light, and very soon the artificial lights troubled me no more than the stars which twinkle in the heavens at night ”

“ An eventful feature in practical lighting is the proper scattering or diffusion of light, by shades, screens, and reflecting surfaces. We want to emulate the diffusion of daylight. It is marvelous how whitewashed surfaces do this. Well-selected globes act as though they were self-luminous; they scatter light and produce shadows. The power of the light, however, to penetrate fogs does not appear to be any greater than that of gaslight. ”

“ Evidently there is some position in which the shadows are equally luminous. When that point is reached, I can read off the current which is passing through the lamp, and having previously standardized it for each increment of current, I know what amount of light is given out. This value of the incandescence lamp I can use as an ordinate to a curve, the scale number which marks the position of the color in the spectrum being the abscissa. ”

“ For as we dropped swiftly down toward that world we saw at last that the faint light which illuminated it was not artificial light, or reflected light, but light inherent in itself, since all the surface of the mighty sphere glowed with the same phosphorescent light, its plains and hills and valleys alike feebly luminous, with the soft, dim luminosity of radio-active minerals. ”

“ A substance is said to be coloured when it allows light of one kind to pass through, but absorbs light of a different kind. A block of pitch is opaque to visible light, but transparent to electric radiation; whereas water, which is transparent to light, is opaque to electric radiation. These substances exhibit selective absorption, and are therefore coloured. ”

“ When white light is decomposed by the prism, we obtain seven groups of visible rays, of unequal refractive power, violet, indigo, blue, green, yellow, orange, and red. The spectrum or ribbon of colors thus obtained widens and spreads out by invisible radiations. Beyond the red, there exist radiations of dark heat, or calorific rays, and, outside of the violet, radiations which are called chemical or ultraviolet rays. The first affect the thermometer, the last occasion energetic reactions in chemical compounds. What is their influence upon vegetation? ”

“ In like manner, according to the wave-theory of light, you have a source of agitation in the vibrating atoms, or smallest particles, of the luminous body; you have a vehicle of transmission in a substance which is supposed to fill all space, and to be diffused through the humors of the eye; and, finally, you have the retina, which receives the successive shocks of the waves. These shocks are supposed to produce the sensation of light. We are here dealing, for the most part, with suppositions and assumptions merely. We have never seen the atoms of a luminous body, nor their motions. ”

“ Thus, a glass or fluid appears yellow to the eye, because it has the property of admitting the permeation of a larger quantity of the yellow rays than of any others;—red, because the red rays pass it with the greatest freedom; and so on for every other colour. In most cases the powers of transmission and of reflection are similar; but it is not so in all; a variety of fluor spar, which, while it transmits green light, reflects blue, and the precious opal, are striking instances to the contrary. ”

“ Reflection of light is essential to the visual perception of substances. We could not see a substance if it did not reflect light—unless it is warm enough to emit radiation in the visible wavelengths. Selective reflection of light wavelengths determines the apparent color of a substance; a substance has a certain color because that wavelength is being reflected and the others absorbed. ”

“ If the light of an electric lamp be caused to form a clear sunlike disk upon a white screen, the gradual change of this light by augmented precipitation into deep glowing red, resembling the colour of the sun when seen through fine London smoke, is exceedingly striking. ”

“ Fading Illumination. The farther we move from a light, the less strong, or intense, is the illumination which reaches us; the light of the street lamp on the corner fades and becomes dim before the middle of the block is reached, so that we look eagerly for the next lamp. The light diminishes in brightness much more rapidly than we realize, as the following simple experiment will show. Let a single candle [106] (Fig. 57) serve as our light, and at a distance of one foot from the candle place a photograph. ”

“ In considering artificial light, we will avoid all efforts to analyze the different forms of energy, magnetic energy, electric energy, heat energy, mechanical momentum, radiating energy, and deal with result rather than with cause and effect. It will be sufficient to state as the deduction of the scientist that certain waves or vibrations which affect the fibers of the optic nerve are transmitted by the brain into color. (¶ 3.) Self-luminous bodies are bodies which produce light. ”

“ It is now found that the chemical rays are more refrangible than the luminous, and that, while the darkening takes place in the colored spectrum, it it strongest in the violet of all the colors, and extends also through the dark space up to B, as shown in the figure. ”

“ Those that exist beyond the red ray are capable of producing heat, while those that are beyond the violet excite chemical action. These invisible violet rays can be actually made perceptible to the eye, or, in other words, the undulations or waves proceeding from this end of the spectrum can be made to strike against certain substances and induce luminous vibrations, so as to connect the dark space beyond the violet with a brilliantly illuminated band. ”

“ Objects like chairs, tables, carpets, have no light within themselves and are visible only when they receive light from a luminous source and reflect that light. We know that these objects are not self-luminous, because they are not visible at night unless a lamp or gas is burning. When light from any luminous object falls upon books, desks, or dishes, it meets rough surfaces, and hence undergoes diffuse reflection, and is scattered irregularly in all directions. No matter where the eye is, some reflected rays enter it, and the various objects are clearly seen. ”