Black bodies are theoretical objects that absorb all incident radiation, regardless of wavelength or frequency, and emit energy at all wavelengths as well. This phenomenon is known as black-body radiation or Planck's radiation law, named after the physicist Max Planck, who first described it.
The reason why black bodies emit energy at all wavelengths is due to the thermal motion of their constituent particles, such as atoms or molecules. According to the principles of quantum mechanics, these particles possess quantized energy levels, and they can absorb or emit energy in discrete packets known as photons.
When a black body is heated, its constituent particles gain thermal energy and become excited. As these excited particles undergo transitions between different energy levels, they emit photons. The distribution of these emitted photons across different wavelengths or frequencies is determined by the temperature of the black body.
The relationship between the temperature of a black body and its emitted radiation is described by Planck's radiation law. It states that the intensity of radiation emitted by a black body at a given wavelength is proportional to the temperature of the black body and inversely proportional to the fourth power of the wavelength. This means that as the temperature increases, the emitted radiation shifts towards shorter wavelengths, and the total energy emitted across all wavelengths increases.
Therefore, at any temperature above absolute zero, a black body will emit energy at all wavelengths, covering a continuous spectrum. The specific distribution of energy across different wavelengths is given by the Planck distribution, which describes the characteristic shape of the black-body spectrum at a given temperature.
It's worth noting that while real objects may not perfectly exhibit the characteristics of ideal black bodies, many objects approximate black-body radiation to varying degrees, depending on their composition and temperature.