BLACK BODY. The term black is commonly applied to any substance, such as coal or pitch, which absorbs nearly all the light falling on it and reflects very little. Lampblack or smoke-black consisting of very fine particles of carbon is usually taken as a typical example of a black substance. The majority of black substances owe their blackness to the presence of particles of carbon embedded in a binding medium, but reflect some light at the surface when smooth or polished. For scientific experiments in which complete absorption of the incident light is required, reflection at the surface must be avoided as far as possible. For this reason the internal surfaces of optical instruments, such as telescopes and cameras, are coated with a "dead-black" varnish, having a matt surface, to eliminate internal reflections which would obscure or fog the image.
In the case of visible light, a very small defect of perfect blackness can be detected readily by eye, but the visible rays form a very small proportion of the total radiation from any source of artificial light, and are completely absent in the case of any radiator at a temperature below a red heat. It cannot be assumed that surfaces which appear black to the eye and are practically perfect absorbers of visible light, are equally perfect absorbers of the invisible rays. Receiving instruments for de tecting or measuring radiation, such as the thermopile or bolom eter (see HEAT), depend for their accuracy on the completeness of the absorption of all radiation over a range of wave-length about a hundred times greater than that of visible light. The sensitive receiving surfaces of such instruments are necessarily metallic, and would be very bad absorbers of radiation. They are accordingly coated with a film of smoke-black or platinum-black to render the absorption as complete as practicable. An absorp tion equivalent to 98% of the incident radiation may thus be obtained under favourable conditions, but it is very difficult to estimate the percentage defect and to make sure that it is the same for all wavelengths. Some allowance is usually made for defective absorption in modern experiments, but it remains one of the chief sources of uncertainty in accurate measurements. The condition of complete absorption cannot be met satisfac torily except by the adoption of the "black body" principle which has long been recognized as essential to the construction of a source of radiation possessing the characteristics of definite in tensity and quality.
The theory of the black body as a perfect absorber is ex tremely simple. The apparatus consists merely of an aperture in the side of an otherwise light-tight chamber, the inside of which is blackened. Any fadiation entering through the hole is scattered and absorbed by repeated reflection so that an infinitesimal frac tion only could possibly escape. It is easy to convince oneself of the perfect blackness thus secured by removing the lens from a box camera and covering part of the aperture with a card painted with the best dead-black. The uncovered part of the aperture will appear of a much higher order of blackness than the most perfect dead-black coating procurable. The advantage of the method lies in the fact that the blackness thus secured is equally perfect for all wavelengths owing to the complete elimination of surface reflection at the aperture.
The practical application of the principle to the construction of a perfect receiver is not quite so easy. With a simple chamber as above described, the radiation entering through the aperture would be scattered over the inner surface and could not easily be collected and measured. The experimental difficulties of this problem cannot be discussed here but are illustrated in the article on HEAT. The application of the same principle to the construc tion of sources of radiation, also termed "black bodies," is de scribed in the same article, but it will be well to explain here how the apparent misnomer "black" has come to be applied to a body emitting radiation.
It follows from the theoretical reasoning of Balfour Stewart (1858) and Kirchhoff (1859), based on the equilibrium of tem perature, that a body capable of absorbing radiation perfectly at any temperature must be equally perfect in the emission of radiation. Further it follows that the intensity of radiation inside a hollow chamber at a uniform temperature, can depend only on the temperature, is the maximum possible at the given tem perature, and is independent of the form of the chamber or of the materials of which it is composed. Chambers of this kind, provided with suitable heating appliances and with apertures for the emission of the internal radiation, are commonly termed black bodies and are employed as sources of radiation of definite quality and intensity. The radiation they emit is the same as that of a perfectly black surface at the same temperature and is often called "black" radiation. The black-body temperature of a source like the sun is the temperature at which a black body would emit radiation of the samc intensity. (H. L. C.)