FIG. 4.
(a) Light force. Laser light with momentum in a single direction is absorbed by an atom and re-radiated in all directions, resulting in a net transfer of momentum to the atom. (b) Laser cooling. The scattering rate is shown for an atom at rest with a laser tuned to the red of atomic resonance. For atoms with a velocity (+v) in a direction opposite to the laser direction, the Doppler shift increases the apparent laser frequency and the laser light is absorbed more strongly compared to atoms travelling with velocity −v. The result is a velocity-dependent force that damps the atomic motion.