Emission theory

“ If an emission theory is accepted, it would seem most natural to assume that the excited portion of a reflecting mirror acts as a new source of light and that reflected light has the same velocity c with respect to the mirror as has original light with respect to its source. ”

“ 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. ”

“ The present century finally established it through the discovery of interference, the destruction of the emission theory being inevitable when it was shown that light, interfering under certain circumstances with light, may produce darkness, as sound added to sound may produce silence—results arising from the action of undulating motion. The difficulties presented by polarization were not only removed, but that class of phenomena was actually made a strong support of the theory. ”

“ It is of importance in molecular and atomic investigations, in osmosis, electrolysis, and in most questions dealing with thermodynamics.
Light is generally defined as the sense impression received by the eye. It was formerly believed that it was caused by streams of corpuscles emitted by the source of light. This was known as the emission theory. Early in the nineteenth century, the undulatory displaced the emission theory. According to this, light is a transverse vibratory motion extended longitudinally through the ether. ”

“ Highly superheated vapors can be treated as gases, and by appropriately mixing different substances one can always imagine a gas mixture being produced which has a finite absorptivity for all wavelengths. But one must not think here of the radiation which the gases emit under the influence of electrical or chemical processes.
If one assumes a gas as a radiating body, Maxw ell's law of the distribution of velocities will apply if we take as our basis the kinetic theory of gases. The absolute temperature will be proportional to the mean kinetic energy of the gas molecules. ”

“ The production of radiant heat is a consequence of the act of emission, and its destruction is the result of absorption. Both processes, emission and absorption, have their origin only in material particles, atoms or electrons, not at the geometrical bounding surface; although one frequently says, for the sake of brevity, that a surface element emits or absorbs. In reality a surface element of a body is a place of entrance for the radiation falling upon the body from without and which is to be absorbed ”

“ According to the present theory we must assume that this absorption is followed by an emission of energy during which the systems pass back to the original stationary state. If there are no collisions between the different systems this energy will be emitted as a radiation of the same frequency as that absorbed, and there will be no true absorption but only a scattering of the original radiation; a true absorption will not occur unless the energy in question is transformed by collisions into kinetic energy of free particles. ”

“ The capacity for emission and the capacity for absorption of an element of a body depend only upon its own condition and not upon that of the surrounding elements. If, therefore, as we shall assume in what follows, the state of the body varies only with the temperature, then the capacity for emission and the capacity for absorption of the body will also vary only with the temperature. The dependence upon the temperature can of course be different for each wave length. ”

“ It is only the presence of material bodies that by their continued emission and absorption can transform the surrounding radiation towards the unique constitution which corresponds to their temperature. We can define the temperature of an isolated field of radiation, of this definite ultimate constitution, to be the same as that of the material bodies with which it would thus be in equilibrium. ”

“ The strong components are [Pg 53] saturated, the weak ones are not. It should be noted that here there is an excess of atoms for the radiation. “Saturation” is used in another sense when the word is applied to the conditions at the center of a star, [116] where there is an excess of radiation for the atoms present.
EMISSION LINES
The emission lines observed in stellar spectra differ more widely among themselves than do the absorption lines, and theory has so far been less successful in suggesting the physical conditions under which they may arise. ”

“ 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. ”

“ Till recently, the theory of material heat, and of its propagation by emission, was probably the one most in favor with those who had studied mathematical thermotics. As we have said, the laws of 182 conduction, in their ultimate analytical form, were almost identical with the laws of motion of fluids. Fourier’s principle also, that the radiation of heat takes place from points below the surface, and is intercepted by the superficial particles, appears to favor the notion of material emission. ”

“ According to the theory of emission, the transmission of energy is effected by the actual transference of light-corpuscules from the luminous to the illuminated body, carrying with them their kinetic energy, together with any other kind of energy of which they may be the receptacles. ”

“ The energy values differ so little from one another that it is impossible to represent them in the figure as separate dots. The emission (and absorption) of the two components of the yellow line are, therefore, connected with two different processes. This was beautifully shown by some later researches of Wood and Dunoyer. They found that if sodium vapour is subjected to radiation from only one of the two components of the yellow line, the resonance radiation, at least at low pressures, consists only of this component. ”

“ But among all the different maxima there is a special maximum, the absolute, which indicates stable equilibrium. In fact, we shall see that in a diathermanous medium for each temperature there exists a quite definite intensity of radiation, which is designated as the stable intensity of radiation of the frequency considered. But for the present we shall assume for all frequencies a finite capacity for absorption and for emission. ”

“ The optical theory which reigned in the scientific world until the beginning of the present century was known as the theory of emission, which assumed that all luminous effects are due to the darting, rebounding, and deflecting of some kind of material corpuscles or particles. ”

“ We consider a free object at rest that emits the same amount of radiative energy in two opposing directions. In doing so, it remains at rest. Let the object's energy prior to emission be denoted , and its energy after emission and let be the energy of the emitted radiation. By the energy principle, we have
Now consider the object and the same emitted radiation from a reference frame moving with velocity relative to the object. Relativity theory gives us the means of calculating the energy emitted in the new reference frame. ”

“ The fraction E measures the emissive power of the body for that particular wave-length, and cannot be greater than unity. The same fraction, by the principle of equality of emissive and absorptive powers, will measure the proportion absorbed of incident radiation R′. If the black body emitting the radiation R′ is at the same temperature as the absorbing layer, R = R′, the emission balances the absorption, and the line will appear neither bright nor dark. ”

“ Inasmuch as matter is the source of all heat and light radiations, the mechanism responsible for the emission and absorption of these radiations must be intimately related to the nature and constitution of matter itself and therefore theories of emission and absorption systems depend to a large extent upon our theories of the nature of atoms and molecules. ”

“ Experience, finally, rectifying the vague predictions to which the most enlightened minds abandon themselves with so little reserve, shows that the calorific rays which emanate from the plane surface of a heated body have not the same force, the same intensity in all directions; that the maximum corresponds to the perpendicular emission, and the minimum to the emissions parallel to the surface.
Between these two extreme positions, how does the diminution of the emissive power operate? Leslie first sought the solution of this important question. ”

“ In the main, these facts may be summed up as the emission of certain radiations from known elemental matter: the material alpha particles with positive charge, the beta particles or negative electrons, and the gamma rays analogous to X rays. The emission of these rays results in the production of great heat. Then there is the law of transformations by which whole series of new elements are generated from the original element and maintain a constant equilibrium of growth and decay in the series. Lastly, we have the production of helium from the alpha particles. ”

“ And, in many cases, the spectra, or portions of the spectra of particular elements, even when present in the most minute proportion, are so extremely well marked and distinctive, that the presence or absence of these elements is determinable with the greatest ease and certainty, by a mere inspection of the emission spectra yielded by the incandescent gases or vapors under examination. Moreover, gases and vapors are further capable of affecting heterogeneous light which is passed through them ”

“ Gases, like atmospheric air, hydrogen or carbon dioxide do not become luminous if they are placed in tubes, even when heated up far beyond white heat as in the electric furnace. This need not necessarily be interpreted as indicating the impossibility of rendering gases luminous by temperature only, for the transparency of the gas for luminous radiations may be such that the emission is too weak to be detected. When there is appreciable absorption as in the case of the vapours of chlorine, bromine, iodine, sulphur, selenium and arsenic, luminosity begins at a red heat. ”

“ Then again, light emission is usually accompanied by many intricate phenomena such as ionization and chemical reactions and this adds to the complexity of the problem. When we consider our profound ignorance respecting even the nature of chemical forces, the constitution of the molecule and atom, the nature of the electric and magnetic fields and even the nature of light itself, it is not at all remarkable that little definite and certain knowledge has been obtained concerning the nature of light centers. ”

“ The collection of observational data as to the activity of liberation of subatomic energy is part of the routine of practical astronomy; and we have to pursue the usual course of arranging the measurements into some kind of coherence, so as to find out how the output is related to the temperature, density, or age of the material supplying it—in short, to discover the laws of emission. ”

“ There are many astronomical indications that the hypothesis attributing the energy of the stars to the transmutation of hydrogen is unsatisfactory. It may perhaps be responsible for the rapid liberation of energy in the earliest (giant) stages when the star is a large diffuse body radiating heat abundantly; but the energy in later life seems to come from a source subject to different laws of emission. ”

“ Gamma--In some decay processes, the emission is a photon having no mass at all and traveling at the speed of light. Radio waves, visible light, radiant heat, and X-rays are all photons, differing only in the energy level each carries. ”

“ It can be simply shown that the energy emitted from a radioactive substance which expels alpha particles is several million times greater than the energy emitted from an equal weight of matter in any known chemical reaction. This emission of energy is mainly to be ascribed to the conversion of the energy of motion of the swift alpha and beta particles into heat, and is thus in a sense a secondary effect of the radiations. ”

“ What is important from the astrophysical point of view is the association of known lines in the spectrum with different levels of energy in the atom, these levels representing definite electron orbits. Absorption and emission of energy take place in an atom by the transfer of an electron from an orbit associated with low energy to an orbit associated with high energy, and vice versa. ”

“ This limitation naturally caused inventors to search for methods by which the emission of light could be obtained from coal-gas otherwise than by the incandescence of the carbon particles contained within the flame itself. ”

“ It appears plausible to assume, at least for low partial pressures, that the amount of energy of any frequency that is lost by black-body radiation in passing through the absorbing layer will vary jointly with the supply of energy and the number of atoms which are in a suitable state to absorb that particular frequency. One of the problems that arise is therefore that of determining what fraction of the whole number of atoms of a given kind will be able to absorb. It is to this problem that ionization theory is able to offer a solution. ”

“ The emission from the carbon points is capable of accurate analysis. To simplify the subject, we will take the case of a platinum wire at first slightly warmed by the current, and then gradually raised to a white heat. When first warmed, the wire sends forth rays which have no power on the optic nerve. They are what we call invisible rays; and not until the temperature of the wire has reached nearly 1,000° Fahr., does it begin to glow with a faint, red light. The rays which it emits prior to redness are all invisible rays, which can warm the hand but cannot excite vision. ”