heat capacity of solids as a function of temperature entering the hole will be reflected by the cavity walls and will be eventually absorbed by the walls. A black body is also a perfect radiator of radiant energy. Furthermore, a black body is in thermal equilibrium with its surroundings. It radiates same amount of energy per unit area as it absorbs from its surrounding in any given time.
The amount of light emitted (intensity of radiation) from a black body and its spectral distribution depends only on its temperature. At a given temperature, intensity of radiation emitted increases with the increase of wavelength, reaches a maximum value at a given wavelength and then starts decreasing with further increase of wavelength, as shown in Fig. . .
Also, as the temperature increases, maxima of the curve shifts to short wavelength. Several attempts were made to predict the intensity of radiation as a function of wavelength. But the results of the above experiment could not be explained satisfactorily on the basis of the wave theory of light. Max Planck arrived at a satisfactory relationship Fig.
. (a) Black body Fig. . Wavelength-intensity relationship (iv) Line spectra of atoms with special reference to hydrogen.
These phenomena indicate that the system can take energy only in discrete amounts. All possible energies cannot be taken up or radiated. It is noteworthy that the first concrete explanation for the phenomenon of the black body radiation mentioned above was given by Max Planck in . Let us first try to understand this phenomenon, which is given below: Hot objects emit electromagnetic radiations over a wide range of wavelengths.
At high temperatures, an appreciable proportion of radiation is in the visible region of the spectrum. As the temperature is raised, a higher proportion of short wavelength (blue light) is generated. For example, when an iron rod is heated in a furnace, it first turns to dull red and then progressively becomes more and more red as the temperature increases. As this is heated further, the radiation emitted becomes white and then becomes blue as the temperature becomes very high.