  The straightforward application of energy and linear momentum conservation to
the absorption/emission of photons by atoms allows to establish the essential
features of laser cooling of two levels atoms at low laser intensities. The
lowest attainable average kinetic energy of the atoms depends on the ratio
$\Gamma/E_R$ between the natural linewidth and the recoil energy and tends to
$E_R$ as $\Gamma$ tends to zero. This treatment, as the quantum mechanical
ones, is valid for any value of the ratio $\Gamma/E_R$ and contains the
semiclassical theory of laser cooling as the limiting case in which $E_R\ll
\Gamma$.
