Abstract
The first lecture was devoted to Einstein's approach in his seminal papers. It shows how the motion of atoms coupled to electromagnetic radiation can be seen as a Brownian process, and why we observe the thermalization of atoms thanks to the Doppler effect. The starting point is blackbody radiation, i.e. electromagnetic radiation emitted by a material body in thermodynamic equilibrium with its environment. The spectral distribution of this radiation, given by Planck's law, is a universal function that depends only on the body's temperature. Taking this law as given, Einstein studied how radiation with this spectral energy density would impose its temperature on a collection of atoms. To do this, he introduced two essential concepts, which we have described in detail:
- a frictional force caused by light on a moving atom ;
- impulse scattering due to the random nature of absorption and emission processes.
