3. Inconsistent sources: black bodies

We saw earlier that electromagnetic waves are radiated by accelerating electric charges. In a heated body, atoms and molecules are subjected to thermal agitation. In gases at low pressure, or under normal conditions of pressure and temperature, molecules are on average several times their size apart, and move freely at high speed $\left(\simeq {10}^6\text{m}\cdot {\text{s}}^{-1}\right)$ between two shocks. In heated solids, atoms vibrate around their equilibrium position. The acceleration of atomic charges due to their chaotic motion is the source of so-called thermal radiation. The properties of this radiation are described...