This is called Black Body radiation spectrum. German physicists Lummer and Pringsheim studied the energy density as a function of wavelength for different temperatures of a black body using a spectrograph and a plot is made. In practice a black body can be realized with the emission of Ultraviolet, Visible and infrared wavelengths on heating a body. Hence when a black body is heated it emits radiations. Spectral Distribution of energy in thermal radiation (Black Body radiation spectrum) A good absorber of radiation is also a good emitter. The absorptive power of a perfectly black body is 1. Absorptive power of a black body: It is defined as the ratio of the total energy absorbed by the black body to the amount of radiant energy incident on it in a given time interval. Emissive power of a black body: It is defined as the total energy radiated per second from the unit surface area of a black body maintained at certain temperature. Platinum black and Lamp black can absorb almost all the radiations incident on them. A perfectly black-body can absorb the entire radiations incident on it. A Body which is capable of absorbing almost all the radiations incident on it is called a black body. The ability of the body to radiate is closely related to its ability to absorb radiation. A hot body emits thermal radiations which depend on composition and the temperature of the body.
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