Ultraviolet light

“ 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? ”

“ So in taking an ultraviolet photograph a screen must be used which will be opaque to these visible rays and yet will let the ultraviolet rays through to form the image. That gave Professor Wood a lot of trouble. Glass won’t do, for glass cuts off the ultraviolet rays entirely. Quartz is a very good medium, but it does not cut off all the visible light. In fact there is only one thing that will do the work, and that is metallic silver.”
I could not fathom what he was driving at, but the fascination of Kennedy himself was quite sufficient. ”

“ After a thousand thousand years, and billions on billions of such repetitions, the handful of photons reached the relatively cool photosphere of the sun. But the long battle had taken some of the drive out of them; over the past million years, even the strongest had become only hard ultraviolet, and the weakest just sputtered out in the form of long radio waves. ”

“ Those too short to be perceived as light are termed as a class "ultraviolet radiant energy." A solid body at a high temperature emits electro-magnetic energy of all wave-lengths, from the shortest ultra-violet to the longest infra-red.
Another complication arises in the variation in visibility or luminosity of energy of wave-lengths within the range of the visible spectrum. Obviously, no amount of energy incapable of exciting the sensation of light will be visible. The energy of those wave-lengths near the ends of the visible spectrum will be inefficient in producing light. ”

“ There is another way in which the existence of both the ultra-violet and infra-red rays can be demonstrated, and that is by means of photography. If we place an ordinary photographic plate in the spectrum and develop it, we shall find that besides being affected by the blue and violet rays, it is also affected by the rays beyond the violet, the energy of these rays being capable of causing a decomposition of the sensitive silver salt. If quartz prisms and lenses be used, and the electric light be the source of illumination, the ultra-violet spectrum will extend to an enormous extent. ”

“ UV-radiation might not be the only part of the light spectrum that is used more in visual communication than previously assumed. For instance, red displays cannot be generated by reflection in depths below 10 to 20 m, because water quickly attenuates the longer wavelengths (>600 nm) of the incoming sunlight [19] . As a consequence, pigments cannot reflect red and will appear grey or black in most of the euphotic zone of many marine environments. ”

“ The material we used should not be darkened or discolored by prolonged exposure to ultraviolet radiation, and it should have good thermal conductance, so that some of the heat absorbed by the solar cells could be conducted out to the cover plates and re-radiated. ”

“ The middle ultra-violet rays are not found in sunlight, because they are absorbed by the atmosphere. They are also absorbed by ordinary glass but are freely transmitted by quartz. The nearer ultra-violet rays are found in sunlight and in most artificial illuminants and are transmitted by ordinary glass. Next to this region is the visible spectrum with the various colors, from violet to red, induced by radiant energy of increasing wave-length. The infra-red rays are sometimes called heat-rays, but all radiant energy may be converted into heat. ”

“ We cause a pencil of white luminous rays to emanate from the incandescent carbon-points of an electric light, which, rendered parallel through a condensing lens, passes through a square glass vessel containing clear water. We introduce a pane of violet glass into its path. The illuminating radiation is cut off by it, and only a few violet rays are transmitted. The ultra-violet ones, however, are represented much more abundantly in the now invisible pencil. Their existence is revealed at once when we add a few drops of the quinine solution to the water. ”

“ All this time its luminous efficiency is increasing, because it is radiating not only an increasing percentage of visible radiant energy but an increasing amount of the most effective luminous energy. But even when it appears white, a large amount of the energy which it radiates is invisible infra-red and ultra-violet, which are ineffective in producing light, so at best the substance at this high temperature is inefficient as a light-producer. ”

“ Heat undulations are subject to the same considerations. It is now apparent that what we call light is the affection of the eye by certain vibrations, the less rapid of which are invisible and constitute the dark rays of radiant heat, in detecting which we must substitute the thermometer or the thermopile for the eye. At the other end of the spectrum, again, the ultra-violet rays are invisible, and only indirectly brought to our knowledge in the phenomena of fluorescence or photo-chemical action. ”

“ JUDGING COLOR UNDER ARTIFICIAL DAYLIGHT
It has been conclusively shown that radiant energy kills bacteria. The early experiments were made with sunlight and the destruction of micro-organisms is generally attributed to the so-called chemical rays, namely, the blue, violet, and ultra-violet rays. It appears in general that the middle ultra-violet rays are the most powerful destroyers. It is certainly established that sunlight sterilizes water, for example, and the quartz mercury-lamp is in daily use for this purpose on a practicable scale. ”

“ 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. ”

“ So much energy is wasted in the production of heat-waves and ultra-violet waves which we do not want, that 90 per cent. or more of the power used in illumination is wasted. Would that the glow-worm, or even the dead herring, would yield us its secret! Phosphorus is the one thing we know as yet that suits the purpose, and—it smells! Indeed, our artificial light is not only extravagant in cost, but often poor in colour. The unwary person often buys a garment by artificial light, and is disgusted next morning to find in it a colour which is not wanted. ”

“ Arc lights throwing powerful ultra-violet rays are being installed in beauty shops and hotels, and patrons are given opportunity to bathe their bodies in this brilliance. These rays are being billed as more potent than sun baths, and citizens who have small chance to see the orb of day get their sunshine and their medicine at one swoop.”
Two Indian scientists, S. S. Bhatnagar and R. B. Lal, of the University of the Punjab, Lahore, discovered in 1925 that germs grow faster when exposed to “polarized” light than to ordinary light. ”

“ So that it may safely be concluded that the areas of the dark and white spaces, respectively, represent the relative energies of the visible and invisible radiation. The one is 7.7 times the other.
But in verification, as already stated, consists the strength of science. Determining in the first place [Pg 188] the total emission from the electric lamp, and then, by means of the iodine filter, determining the ultra-red emission; the difference between both gives the luminous emission. In this way, it is found that the energy of the invisible emission is eight times that of the visible. ”

“ Every clap of thunder means a shower of them. Every star is flooding space with them. We are witnessing the spontaneous breaking up of atoms, atoms which had been thought to be indivisible. The sun not only pours out streams of electrons from its own atoms, but the ultra-violet light which it sends to the earth is one of the most powerful agencies for releasing electrons from the surface-atoms [Pg 258] of matter on the earth. It is fortunate for us that our atmosphere absorbs most of this ultra-violet or invisible light of the sun—a kind of light which will be explained presently. ”

“ Before considering luciferin and luciferase, however, let us study a few well-known chemiluminescent oxidations with special reference to concentration of reacting substances and temperature. [Pg 145]
The effect of temperature on luminescence is of special interest because it gives us a means of analysis for determining if the luminescence depends on reaction velocity. We know that photochemical reactions are very little affected by temperature because the reaction is dependent on the absorption of light, a physical process, and this increases only a small per cent. ”

“ Thus, while the incandescent electric bulb is a great advance over fire, it is still a very inefficient source of light. Because it depends upon incandescence, a considerable portion of the electrical input goes into the production of unwanted heat, for the bulb’s filament radiates in the infrared as well as the visible region. ”

“ On the other hand, we should expect photogenic reactions, in which some of the chemical energy is converted into radiant energy, to give off much more light the greater the reaction velocity. As reaction velocity increases so rapidly with temperature (Q10 = 2 to 3) , luminescence intensity should rapidly increase with increase in temperature. ”

“ A lens made of glass will, but this lens is made of rock crystal, which is readily permeable to ultra-violet. The net result of this apparatus is that we can direct before us as we move in the tank a beam of light which is composed solely of the ultra-violet portion of the spectrum. ”

“ When they impinge upon a metal refractory enough to bear their shock without fusion, they can raise it to a heat so white and luminous as to yield, when analyzed, all the colors of the spectrum. In this way the dark rays emitted by the incandescent carbons are converted into light rays of all colors. Still, so powerless are these invisible rays to excite vision that the eye has been placed at a focus competent to raise platinum-foil to bright redness without experiencing any visual impression. ”

“ Further Photo-Electric Experiments.
Plates of platinum, silver, copper need exceedingly ultra-violet light before they show any photo-electric power; zinc, aluminium, magnesium show it for visible violet and blue light; the alkali metals, in an atmosphere of rarefied hydrogen, advance their range of sensibility into the spectral red; while under the most favourable conditions they show a sensibility only inferior to that of the eye itself. ”

“ We only know it is a fact.
All these ether rays may produce heat. The high-frequency rays affect the photographic sensitive plate, and also produce another effect that is of especial interest in connection with the study of the Röntgen rays. They have the power of exciting a peculiar luminosity, or light, in certain substances, which are for that reason called fluorescent.
Electrical discharges in vacuum tubes have long been known as sources of radiations which produce heat and affect the eye. ”

“ We now come to another very important discovery, made by means of our prism and narrow slit—the determination of the composition of the photosphere or mass of luminous vapour surrounding the body of the sun.
A simple experiment will show how this brilliant discovery was arrived at. The light of a candle or other flame containing incandescent solid matter is passed through the spectroscope, and is found to decompose into a continuous spectrum, uninterrupted by dark lines. Between the light and the slit a spirit-lamp is placed, but no difference in the appearance of the spectrum is perceived. ”

“ It may be some other form of solar radiation to which we are not accustomed on the earth, but which is specially effective here when the sun is uncovered because of the greater nearness of Venus. This atmosphere, notwithstanding its density, may well be diaphanous to the ultraviolet rays, owing to some peculiarity in its composition which I have not had time to study. At any rate, it is evident, from what we have seen, that the rays of the unclouded sun almost instantly affect the brain. ”

“ These are often designated chemical or actinic rays in order to distinguish the group as a whole from other groups such as ultra-violet, visible, and infra-red. Light is a unique agent in chemical reactions because it is not a material substance. It neither contaminates nor leaves a residue. Although much information pertaining to photochemistry has been available for years, the absence of powerful light-sources emitting so-called chemical rays in large quantities inhibited the practical development of the science of photochemistry. ”

“ Light may have great brilliancy and yet be undesirable from its color; we are all familiar with the havoc that gas light may play with hues of blossom and leaf that in sunshine are beautiful. Through ages untold the human eye has been seeing by rays from the sun, and from immemorial habit is best served by light of similar quality. A simple instrument, the spectrometer, casts upon a screen the spectrum from a mercury tube, a Nernst lamp, a Welsbach mantle, or other illuminant, and enables us to compare it with the spectrum of sunshine. ”

“ 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. ”