Why does metal glow when hot

Warming iron or steel to a temperature of about 500 °C causes it to glow a dull red colour, as seen on an electric cooker set at Tow . The oven ring appears bright orange if the temperature increases further ( 1000°C). In these kitchen items, an electric current inductively heats a coil of wire. 

But not all materials emit the same amount of light when heated to the same temperature there is a spectral distribution of electromagnetic waves. For example, a piece of glass and a piece of iron when heated in the same furnace look different the glass is nearly colourless yet feels hotter to the skin because it emits more infrared light conversely, the iron glows because it emits visible as well as infrared light. 

This observation illustrates the so-called rule of reciprocity a body radiates strongly at those frequencies that it is able to absorb, and emits weakly at other frequencies. 

The heated body emits light with a spectral composition that depends on the material s composition. That observation is not the case for an ideal radiator or absorber ideal objects will absorb and thence re-emit radiation of all frequencies equally and fully. A radiator/absorber of this kind is called a black body, and its radiation spectrum is referred to as black-body radiation, which depends on only one parameter, its temperature, so a hotter body absorbs more light and emits more light. 

Strictly, a black body is defined as something that absorbs photons of all energies, and does not reflect light. Furthermore, a black body is also a perfect emitter of light. A black body is a theoretical object since, in practice, nothing behaves as a perfect black body. The best approximations are hot objects such as red- or white-hot metals. 

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