RADIATION. The term radiation is ap plied to a variety of cases in which energy is propagated through space like rays of light. The case most studied is that of heat. Heat rays are regarded as of the same character as light rays, the two merely differing in wave length or frequency. Writers sometimes speak even of the gcoloro of a heat ray to indicate its wave length or period. Conversely, every light ray is simultaneously a heat ray. According to Maxwell's theory these rays are all electromag netic in character, consisting of states of elec tric and magneticintensity perpendicular to the ray and propagated through space with the velocity of light.
Heat conduction in a medium proceeds from points of higher to points of lower tempera ture. If the temperature is constant, there is no conduction. Radiation is, however, inde pendent of the temperature; for instance, the sun's rays can be concentrated by means of a lens of ice to a focus when the temperature is much higher than that of ice. Furthermore the rays passing through a point do not necessarily proceed in the same direction. They may pro ceed simultaneously in many different direc tions. Even the radiations in two opposite di rections are quite independent; to specify the radiation completely, the intensity (rate at which energy flows across unit surface perpen dicular to the direction of motion) must be known in all directions.
The creation of a heat ray is denoted by the word emission. Since this requires energy, only material particles can emit radiation. When we speak of the surface of a body as radiating heat, we mean merely that it allows part of the rays corning from the interior to pass through, The radiation emitted by a substance de pends on the substance and its physical condi tion. With the exception of phenomena of luminescence, it is generally assumed that the radiation emitted by a given body depends only on its temperature and is otherwise independent of the presence or absence of other bodies or fields of radiation.
Heat rays are destroyed by absorption. They are then converted into other forms of energy. This can be accomplished only by material particles. Rays of different frequency
may be absorbed very unequally, but it is as sumed that the fractional part of the rays of each frequency absorbed in penetrating a given distance through a given medium is independent of the intensity of the radiation.
A transparent, or diathermanous body it one through which the rays pass practically un changed. If the rays pass freely but are de flected, or scattered, the body is turbid. If the surface is smooth, the rays may be regularly reflected as by a mirror. If the surface is rough, the rays may still be practically all re fleeted, but irregularly in all directions. The body is then white. A. black body is one that absorbs all the incident radiation.
Normal, or Black, Radiation.—The sim plest type of radiation is that which occurs in an enclosure at constant temperature. The total radiation from a given body under these conditions is called normal radiation.
Consider a number of bodies in an enclosure with perfectly reflecting walls. These bodies exchange heat until they settle to a uniform temperature. They then continue to exchange beat by radiation but each receives just as much as it gives out (Prevost's principle of ex changes). The composition of the resulting radiation is independent of the nature of the bodies present. For, if it were not so, the in troduction of a new body would change the composition of the radiation in the enclosure and, since the emissive power of each body is a function of its temperature only, the resulting changes in the amount of radiation absorbed would cause changes of temperature in contra diction to the second law of thermodynamics.
For a region in temperature equilibrium, ac cording to Kirchoff's law, a body not only re ceives the same total energy it gives out; but if absorbs precisely the same amount of each wave length that it emits; for the radiation received and given out are both normal radia tion. If any radiation is absorbed it must be replaced by precisely the same radiation in emission. It follows, for example, that a body which does not a particular wave length cannot absorb radiation of that wave length and conversely.