12.1 Introduction
In this chapter, we shall study the laws of thermal radiation only. It is the type of radiation the quality and quantity of which depends solely on the temperature of the body emitting radiation. Here, by quality we mean the wavelengths of the radiation emitted and by quantity we mean the energy associated with each wavelength. Thus when a feeble electric current is sent through the wire of an electric heater, we first have a feeling of warmth which gradually increases in intensity as the strength of the current is increased. The wire begins to emit visible light radiation in addition to the invisible heat or infrared radiation, when the temperature of the wire rises to about 550°C.
At this temperature, the wire appears to be ...
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