Dark Red, Hot Red, or White

The story started at the end of the nineteenth century, at that time physicists were desperately chasing a law that would give quantitative indications about the radiation that is emitted by a heated body, a heated horseshoe, for example. After having identified the notion of electromagnetic radiation, it was pointed out that the power radiated from a heated body considerably increases with temperature while, at the same time, the spectral composition extended further and further toward shorter wavelengths. It was also noticed that the amount of emitted power depends on the special body under consideration, this is the reason why physicists invented an ideal object called a "blackbody." This can be thought of as a cavity almost completely isolated from the external world with which electromagnetic energy can only be exchanged through a tiny hole O, see Figure 9.6. The inner walls of the blackbody are perfectly reflecting, any ray penetrating inside will be almost indefinitely reflected, with a low probability of escaping through the aperture O. For an external observer the hole looks like a source of radiation.

Figure 9.6: Schematic representation of a blackbody. Once it has penetrated inside the blackbody a light ray is almost indefinitely reflected on the perfectly reflecting walls, the probability that it could escape being small. A small amount of light can however escape through the small aperture O, which appears to some external observer as a point source...