Radiant energy

“ All substances at temperatures above absolute zero (-469° F) , produce radiant energy. On earth, absolute zero can be approached only in the laboratory and with special apparatus. For all practical purposes, then, we can consider that all materials we ordinarily deal with are radiating energy—even cold ones. ”

“ Radiant energy travels outward in all directions from the emitting substance until it encounters something capable of absorbing it. The absorbed radiation increases the molecular activity in the substance, thereby increasing its temperature and the amount of heat it contains—this is why we frequently sense radiation as heat. The heated substance radiates energy too, and this energy can be absorbed by other substances and converted to heat. ”

“ All of the radiant energy arranged in the order of wavelength forms the electromagnetic spectrum of radiant energy.
Most radiant energy is invisible, and its wavelength and often its presence can be detected only with special instrumentation. Visible light occupies a narrow band near the middle of the spectrum, and its wavelengths can be readily distinguished because light waves of differing lengths are visible as different colors. Radiation from the sun includes all visible wavelengths. ”

“ It is a curious question to ask what becomes of the radiant light from the sun that is not absorbed either by the planets of our system, or by any of the stars. We can only reply to such a question, that as far as we can judge from our present knowledge, the radiant energy that is not absorbed must be conceived to be traversing space at the rate of 188,000 miles a second. ”

“ Cavor's search was a substance that should be "opaque"—he used some other word I have forgotten, but "opaque" conveys the idea—to "all forms of radiant energy." "Radiant energy," he made me understand, was anything like light or heat or those Röntgen rays there was so much talk about a year or so ago, or the electric waves of Marconi, or gravitation. All these things, he said, radiate from centres and act on bodies at a distance, whence comes the term "radiant energy." Now almost all substances are opaque to some form or other of radiant energy. ”

“ If we knew the relation between mass energy and radiant energy, we could give the ratio between the permanent energy of mass of the stars and the luminous energy of the ether. For example, if the mass energy of a star is on the average times its radiant energy, then the total energy of the universe is always equally divided between ether and matter, because the same radiation comes forth from unit volume of matter, and is distributed to units of ether. ”

“ Postulating the validity of the fundamental conservation laws mentioned above, we shall need in the following development only this one cardinal assumption, that a beam of radiation possesses not only momentum and energy, but also mass, travelling with the velocity of light, and that a body absorbing radiation is acquiring this mass as it also acquires the momentum and the energy of the radiation. Therefore a body which absorbs radiant energy increases in mass. ”

“ But because radiation intensity increases with temperature, the warmer object therefore loses heat, and its temperature decreases—part of its heat is transferred to the other object. Conversely, the cooler object receives more radiation than it is emitting and its amount of heat and temperature increases. Eventually, both objects attain the same temperature—their radiant energy gain and loss just balances. Thus, if a substance receives more radiation than it is losing, it gains heat and becomes warmer, but if it is radiating more energy than it is receiving, it loses heat and becomes cooler. ”

“ There's a band of radiant energy somewhere between ultrasonics and radiant heat that hits fast and goes deep, and comes out just as fast, and it gets triggered off by whatever this is that happens with near-absolute zero objects subjected to room temperature. ”

“ Through all these ages the sun must have been pouring out his radiant energy at at least his present rate. Radiation of heat from the earth may have been relatively less rapid from a denser carbon-laden atmosphere in times past than at present, but it never could have been more so. ”

“ The electromagnetic waves of radiant energy move only along straight paths. Hence, the intensity of radiation received depends on the angle of the incoming radiation and the distance from the source. Radiation perpendicular to the receiving surface is most intense. For a point source of radiation the intensity decreases inversely with the square of the distance from the source. ”

“ Radiation strikes the receiving object from all of the points—in greatest intensity from points perpendicular and closest to the object, and in lesser intensity from the other points. Because so many points are producing radiant energy, the decrease in intensity with distance from a flame source is less than that from a line source and much less than that from a point source. The number of points from which radiation can be received also increases rapidly with the area of the radiating surface. ”

“ The evolution of heat from radium and the radio-active substances is, in a sense, a secondary effect, as it measures the radiant [Pg 35] energy transformed into heat energy by the active matter itself and whatever surrounds it. Let us repeat, therefore, that the total amount of energy pent up in a single atom of radium almost passes our powers of conception. ”

“ Much of the organized matter which the blazing suns break down and disperse as radiant energy was originally built up in these early appearing hydrogen clouds of space. Under certain unusual conditions atom disruption also occurs at the nucleus of the larger hydrogen masses. And all of these phenomena of atom building and atom dissolution, as in the highly heated nebulae, are attended by the emergence of flood tides of short space rays of radiant energy. ”

“ I cannot quite reconcile such a doubt with faith in the principle of continuity. But it does seem to me needful, before we conclude that radiated energy is absolutely and forever wasted, that we should find out what becomes of it. What we call radiant heat is simply transverse wave-motion, propagated with enormous velocity through an ocean of subtle ethereal matter which bathes the atoms of all visible or palpable bodies and fills the whole of space, extending beyond the remotest star which the telescope can reach. ”

“ Above all, we are concerned here with the determination of those quantities which at any place in a medium traversed by heat rays determine the state of the radiant heat. The state of radiation at a given place will not be represented by a vector which is determined by three components; for the energy flowing in a given direction is quite independent of that flowing in any other direction. In order to know the state of radiation, we must be able to specify, moreover, the energy which in the time flows through a surface element for every direction in space. ”

“ In asking to be included in this number, let me add that to me the criterion of this advance is not in any ratiocination, not in any a priori truth, still less in the dictum of any authority, but in the undoubted observation that our doctrine of radiant energy is reaching out over nature in every direction, and proving itself by the fact that through its aid nature obeys us more and more ”

“ It also holds good for phosphorus burning in oxygen. Every gush of dazzling light has associated with it a gush of invisible radiant heat, which far transcends the light in energy. This condition of things applies to all bodies capable of being raised to a white heat, either in the solid or the molten condition. It would doubtless also apply to the luminous fogs formed by the condensation of incandescent vapours. ”

“ A radiator for heating a room should therefore have a dull, dark surface, while a vessel which is designed to keep its contents from losing heat should have a highly polished exterior.
A perfectly transparent substance would radiate no energy, whatever the temperature to which it is {42} raised, for its absorbing power is zero and therefore its radiating power is also zero. No perfectly transparent substances exist, but some substances are a very near approach to it. ”

“ Now it has been shown (section 266) that one gram of radium emits during its life an amount of heat corresponding to 1·6 × 109 gram-calories. It has also been pointed out that there is every reason to suppose that a similar amount of energy is resident in the chemical atoms of the inactive elements. It is not improbable that, at the enormous temperature of the sun, the breaking up of the elements into simpler forms may be taking place at a more rapid rate than on the earth. ”

“ When a beam of radiation travels without suffering absorption, its temperature thus continues to be that of its source multiplied by the coefficient of emission of the source for that kind of radiation, this coefficient being less than unity except in the case of a perfect radiator; but when its intensity I falls by δI in any part of its path owing to absorption or other irreversible process, this involves a further fall of temperature of the energy of the beam and a rise of entropy which can be completely determined when the relation connecting μ−3Eλ with T and λ is known. ”

“ This effect is due to the α and β rays from the radium, and not to the luminous rays from it. Since energy is required to produce ozone from oxygen, this must be derived from the energy of the radiations. ”

“ The rapid heat emission of radium follows naturally from the disintegration theory of radio-activity. The heat is supposed to be derived not from external sources, but from the internal energy of the radium atom. The atom is supposed to be a complex system consisting of charged parts in very rapid motion, and in consequence contains a large store of latent energy, which can only be manifested when the atom breaks up. For some reason, the atomic system becomes unstable, and an α particle, of mass about twice that of the hydrogen atom, escapes, carrying with it its energy of motion. ”

“ Curie and Laborde*8* made the remarkable announcement that a crystal of radium is persistently warmer than its surrounding medium; in other words, that it is perpetually giving out heat without apparently becoming cooler. At first blush this seemed to contradict the great physical law of the conservation of energy, but physicists were soon agreed that a less revolutionary explanation of the phenomenon is perfectly tenable. The giving off of heat is indeed only an additional evidence of the dissipation of energy to which the radio-active atom is subjected. ”

“ The rays are unequally absorbed by different kinds of matter, the absorption varying approximately as the density. In photographs of the hand the bones do not stand out as in X ray photographs.
Curie and Laborde have shown that the compounds of radium possess the remarkable property of always keeping their temperature several degrees above the temperature of the surrounding air. Each gram of radium radiates an amount of energy corresponding to 100 gram-calories per hour. ”

“ Following the preparatory considerations of the last lecture we shall treat today the problem which we have come to recognize as one of the most important in the theory of heat radiation: the establishment of that universal function which governs the energy distribution in the normal spectrum. The means for the solution of this problem will be furnished us through the calculation of the entropy of a resonator placed in a vacuum filled with black radiation and thereby excited into stationary vibrations. ”

“ In recent years numerous researches have supplied valuable materials which ought to be utilized by those endeavouring to construct a theory of radiation. We are, perhaps, still ill informed as to the phenomena of luminescence in which undulations are produced in a complex manner, as in the case of a stick of moist phosphorus which is luminescent in the dark, or in that of a fluorescent screen. But we are very well acquainted with emission or absorption by incandescence, where the only transformation is that of calorific into radiating energy, or vice versa. ”

“ In addition, it was found that the ray products, viz. the emanation radium A and radium C, each gave a heating effect approximately proportional to their activity. Measurements have been made on the heating effect of uranium and thorium and of pitchblende and polonium. In each case, the evolution of heat has been shown to be approximately a measure of the kinetic energy of the α particles.
Experiments on the evolution of heat from radium and its emanation have brought to light the enormous amount of energy accompanying the transformation of radioactive matter where α particles are emitted. ”

“ The radium would thus be heated by its own bombardment above the temperature of the surrounding air. The energy of the expelled α particles probably does not account for the whole emission of heat by radium. It is evident that the violent expulsion of a part of the atom must result in intense electrical disturbances in the atom. At the same time, the residual parts of the disintegrated atom rearrange themselves to form a permanently or temporarily stable system. During this process also some energy is probably emitted, which is manifested in the form of heat in the radium itself. ”

“ With the emission of radiation it can go from stationary states with greater energy to others with less energy, ending its journey by remaining in the orbit which corresponds to the least possible energy. ”