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40 chemistry (iv) Line spectra of atoms with special entering the hole will be reflected by the cavity reference to hydrogen. walls and will be eventually absorbed by the These phenomena indicate that the system walls. A black body is also a perfect radiator of can take energy only in discrete amounts. radiant energy. Furthermore, a black body is All possible energies cannot be taken up or in thermal equilibrium with its surroundings. It radiates same amount of energy per unit radiated. area as it absorbs from its surrounding in Itis noteworthy that the first concrete explanation for the phenomenon of the black any given time. The amount of light emitted body radiation mentioned above was given (intensity of radiation) from a black body and its spectral distribution depends only by Max Planck in 1900. Let us first try to on its temperature. At a given temperature, understand this phenomenon, which is given intensity of radiation emitted increases below: with the increase of wavelength, reaches a Hot objects emit electromagnetic maximum value at a given wavelength and radiations over a wide range of wavelengths. then starts decreasing with further increase of At high temperatures, an appreciable wavelength, as shown in Fig. 2.8. Also, as the proportion of radiation is in the visible temperature increases, maxima of the curve region of the spectrum. As the temperature shifts to short wavelength. Several attempts is raised, a higher proportion of short were made to predict the intensity of radiation wavelength (blue light) is generated. For as a function of wavelength. example, when an iron rod is heated in a But the results of the above experiment furnace, it first turns to dull red and then could not be explained satisfactorily on progressively becomes more and more red as the temperature increases. As this is the basis of the wave theory of light. Max heated further, the radiation emitted becomes Planck arrived at a satisfactory relationship white and then becomes blue as the temperature becomes very high. This means that red radiation is most intense at a particular temperature and the blue radiation is more intense at another temperature. This means intensities of radiations of different wavelengths emitted by hot body depend upon its temperature. By late 1850’s it was known that objects made of different material and kept at different temperatures emit different amount of radiation. Also, when the surface of an object is irradiated with light (electromagnetic radiation), a part of radiant energy is generally reflected as such, a part Fig. 2.8  Wavelength-intensity relationship is absorbed and a part of it is transmitted. The reason for incomplete absorption is that ordinary objects are as a rule imperfect absorbers of radiation. An ideal body, which emits and absorbs radiations of all frequencies uniformly, is called a black body and the radiation emitted by such a body is called black body radiation. In practice, no such body exists. Carbon black approximates fairly closely to black body. A good physical approximationtoablackbodyisacavitywitha tiny hole, which has no other opening. Any ray Fig. 2.8(a)  Black body Reprint 2026-27