The discovery and containment of fire enabled man in the early days of civilisation to bring light and warmth into his dwelling, which meant he no longer had to rely completely on the rays of the sun. This type of light supply is still very common today: Solid matter is heated to very high temperatures and it then radiates – among others – visible light.

For example, when a candle is lit soot particles are formed, which burn and release the yellow light of the candle’s flame, creating a luminous zone around the wick. Or the tungsten filament in the centre of a light bulb is heated by the electric current to such a high temperature that the tungsten gets “white hot”, and that heat emits a great deal of visible light. The hotter the soot particles or filament become, the more visible light they emit.

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